I will continue to edit this as I have time.

After answering many of the same questions many times over I decided to make a thread that lays out the basics. This will give people a good starting point to head out on their own and find more info and ask more educated questions.
As always the “use the search button” suggestion still rings true. There is a lot of good information out there and the more you find the more knowledgeable you will be. It helps a lot getting different peoples perspective and sometimes one persons explanation of something may help you understand better than anothers. There is also a lot of bad information out there as well as outdated information that was the best we had at the time but is no longer pertinent or perhaps even accurate. Use your best judgment out there to decide what is what. If you ever question any info or want to know more about it then ask.
It is also very beneficial to  post up and lay out a game plan before you act. The research before hand feeds you a great amount of info and keeps cranky old farts (even if  only metaphorically old) from needing to answer the same old questions over and over again. Laying out your game plan before acting can give us a chance to catch any bits of misinformation or outdated beliefs or concepts. This will help you get to your goal of most power, fun, happiness and success, with the least amount of money, frustration and failure.
The best places I know of for finding information on boosting the 4AGE are
www.mr2oc.com
www.club4ag.com
www.mr2.com

I am active on all those forums under the name yoshimitsuspeed and I try to help answer questions as much as possible. I used to be most commonly found on MR2OC.com until it was bought by Autoguide and became a corperately owned site with profits being their primary driving force. I am now partial to the MR2.com forums.
There are also some good books on general turbo theory and implementation.
The one I know of personally is Maximum Boost by Corky Bell. It was written a long time ago and has some outdated information in it as well but it's still a good primer on the subject. I know there are other very good ones out there as well but don't have personal experience with them. I will add more if other people recommend them.

One of the most common questions is how much boost can a stock 4AGE take or how much boost can this setup or that setup take.
This is an impossible question to answer and more importantly it is not the important question to be asking. Boost does not make HP, boost is the air that doesn't make it into the engine. Mass of air moving through the engine and the efficiency at which the engine moves that air through it along with basics like fuel and timing is ultimately what makes power.  How much boost an engine can take is dependant on so many variables that it can't even come close to being answered with out some of the variables filled in. A stock internals NA 4AGE might be able to run 30 PSI boost if it was running race gas with meth injection and a perfect tune. You could just as easily blow it up at  6 PSI  if you did something wrong.

Gauges and sensors
Different gauges will have different levels of importance for different builds.

Wideband AFR gauge
 In my opinion this is vital for any build beyond light mods. NA or turbo if you notably effect the amount of air going through your motor you will want to be able to monitor the ratio of fuel going through the motor.
We have a range of gauges available. What is on our site is only a fraction of what we could get since it would take a rediculous amount of time adding all the gauges we can get from Autometer, AEM, Defi, Innovate and others. If you don't see it on our site hit us up. We can probably get it. 
http://matrixgarage.com/store/wideband-gauges

Knock monitor

 This is the second most important gauge for tuning. If you are doing a low comp GZE longblock at 10 PSI with conservative timing this may not be a very vital gauge.  If you are doing a turbo build on a  stock NA 4AGE this is a pretty important gauge. For one the NA ECU doesn't have a knock sensor so if it does get detonation the ECU can't do anything about it. Second you are running boost on higher compression pistons that are relatively weak. Though the NA pistons have been known to take a bit of boost they are not forgiving at all when it comes to knock.
Running boost on an NA smallport, silvertop or blacktop with stock internals a knock sensor will be all the more vital due to the very high compression that they run. I would not consider those to be very good beginner builds due to the very narrow margin for error you will have before something pops. With the right attention to detail they can all be boosted though. 
I call it a gauge out of simplicity but there are a number of options from DIY detcans at  the $20-$50 range, knock lights in the $150-$250 range such as the knocklink  all the way up to the J&S saeguard which is able to actually pull timing to save your motor. Finding what is the right balance of cost to security will be a choice for you to make. There are also proffessional detcans such as the Haltech Knock Ears. Detcans are known to be one of the best knock monitoring systems however the J&S has a headphone port and for the price it's the easy choice for me.

On a largeport NA turbo I would highly recommend this even for a 6-8 PSI build. I would say it's essential for anything over that.
For  the smallport or 20 valve 4A-GE I would say some form of knock monitoring is essential before you throw any positive air pressure at them.
It may seem expensive now but compare the cost to that of blowing up a motor and these components seem very cheap.

Last is a pyrometer or EGT (Exhaust Gas Temp) gauge. . Again if you are doing a GZE longblock with low boost and a Conservative tune this is not a very vital gauge. The more you plan to push your tune the more important it becomes.
A pyrometer is the most important with the high compression builds because retarding timing leads to higher EGTs. Higher EGTs lead to pistons and valves melting. In a high compression build you may have to retard the timing to keep the engine from knocking. As you do this you need to monitor your EGTs to make sure you don't start melting things.

On a NA  largeport at 6 or so PSI I would say this isn't a terribly important gauge. As long as you are able to keep the stock timing and good AFRs your EGTs are likely fine. If you want to start pushing the NA pistons and or ECU further than that it will start becoming more important.
On a turbo SP or 20v build I would consider this vital for any level of boost.

Now these are just suggestions and many people have ran aggressive builds without them. Some successfully, some not so much. The question you want to ask yourself is if you would rather risk it and  have much greater potential for serious and expensive engine damage or if you would rather play it safe and spend a few hundred dollars on monitoring equipment  to greatly reduce your chances of doing big damage.

Next let's talk about internals.
People will often ask what do I need to make X power or run X boost.
The first answer and only one that's easy to answer is a good tune.
So I will start with one of my favorite 4A-GTE quotes from John Welch of http://www.wcengineering.com/. John has been boosting 4A-GEs and getting very impressive power numbers out of them for a long time.
"I ran a bone stock 3 rib block with about 60k miles on it (Bone stock, 9.4:1 compression, stock oil pump, stock, stock, stock !!!  as removed from a 1986 MR-2 that was in a front end wreck, not opened, no seals replaced, no rings replaced, NOTHING REPLACED ! STOCK ! ) Engine ran great.  Took 4th place in my race with about 30 cars in my class.

When I installed the engine (Late April 1994) I had been running a fully built 8.0:1 engine.  I adjusted the timing a couple of degrees, and dropped the boost to 15 psi.  I needed it to last for ONE race and that was all I expected to get out of it, it kept running, every race after that I expected to be it's last..  and it kept running.  Please don't ask for a horsepower number, because that engine was never on a dyno.

After I pulled it out, I gave it to a friend that was doing local oval racing in a Corolla, running it N/A  He ran it four seasons before an oil related failure.

So, the question was:

"How much boost and hp can a 3 rib block hold?"

Answer:

It all depends on your budget and tuning."

The three rib block is the weakest 4A block. It has the weakest crank, some of the smallest rods and weak pistons.
Now this isn't to say everyone should try to do this, it just stresses the importance of a good tune and shows what can be done with one. People have blown up much more robust builds with much less boost.

Since they are the biggest weakness in all NA 4AGEs we will start with pistons.
As seen above the NA pistons can take a decent amount of boost if tuned right.
If someone wants to get into boost on a budget and has a healthy motor I will often recommend starting on the NA pistons. I will also recommend planning on starting with low boost and building up as your knowledge and skills grow. You can decide to play it safe or you can decide to see what it will take before something lets go. With the right tuning and monitoring you would likely be quite surprised how much it will take.
If you are already going into the motor for a rebuild or building it from the ground up then you should just start with good pistons. The OEM 4AGZE pistons have proven as a good choice. People have made a lot of power on them. The downside is the largeport GZE pistons are 8.1:1 compression. Even for boost that is very low by todays standards.
I am a fan of high compression. The more compression you run the more power per lb of boost you will make. It also improves off boost power, spool and gas mileage. It also means the turbo doesn't have to work as hard and in some situations allows you to run a smaller quicker spooling turbo.
For 200-250 WHP onpump gas I would like to see in the range of 9.5:1-10.5:1 depending on cams, tune and how comfortable you are pushing the limits.
For 250-300 HP I would probably come down to 9.5:1 to 10:1. At this point cams will become much more important and bigger cams will allow you to run a little higher static compression ratio. Getting to say 300 Hp one person might choose running 9.5:1 compression and stock cams with 30 PSI boost. Another person might choose to do it with 12:1 compression, 296 cams and 7 PSI boost. Personally I do not believe the former is a good route. most will likely choose something inbetween these two extremes. 

When it comes to aftermarket pistons we have quite a few available. The CP 9:1 pistons aren't bad for low compression setups. They could be taken a little higher with head shaving. 

CP 9:1 16 valve 4A-GE

CP 9:1 20 valve 4A-GE
 

For the last six months we have been working very closely with Arias on their 16 valve pistons and I believe that they are now by far the best design available off the shelf. We can also make custom pistons using everythign that we have learned in developing these pistons. 
The best part is that these pistons are also a great price. 
For a booste build you would most likely choose the 9.5:1 pistons or the 10.5:1 pistons. You can then shave the head to hit your desired compression ratio. 

One thing you should pay close attention to is squish gap which is the distance from the piston to the squish band on the head. The closer you can get this to about .6 to .8mm the better off you will be. Therefore you don't want to use thicker headgaskets to lower compression. Choose your piston to get you as close to your target as possble. Shave the head a little if you want a little higher. Then choose your headgasket thickness to set your desired squish gap. 

The rest of the 4AGE is notoriously robust. The original 3 rib is the lightest of the generations including a smaller lighter crank and smaller rods. This motor still rarely sees inexplicable failure even when pushed far beyond it's original design criteria.  It should be noted though that the early crank and rods will not work with later pistons including GZE pistons and the above aftermarket pistons.. If you want to run later pistons with the 20mm wristpin you will need the later crank and rods and at that point you might as well get a 7 rib shortblock or longblock as the block is also stronger. If you can find a 4AGZE shortblock or longblock for a decent price then that can be a great route. If you can only find a 7 rib NA block remember the only difference between the 4AGE and the 4AGZE is the pistons. There are a couple ancillary differences. For example most NA blocks don't have the bung for the knock sensor drilled. They do all have the bung though so if you need a knock sensor you can drill and tap it yourself. Just be careful not to drill through the block.

The one other motor that has small rods is the 20v blacktop. Many people will tell you not to run those rods in a boosted motor. They will tell you this because the rods look small and people assume that boost is hard on rods.
Normal amounts of boost are not hard on rods. As explained in Maximum Boost, the most stressful time in a rods life is under tension on the exhaust stroke as the piston is forced to a halt at the top of the cylinder and yanked back in the other direction by the rod. This force is so great that on the compression stroke the air and fuel in the cylinder actually acts as a cushion and reduces the tensile stress on the rod. The one time this may not apply is under detonation or preignition where cylinder pressure builds too early. So the strength requirements of the rod comes more importantly from how fast you want to spin the engine. If you want to rev above 8500 you might want to replace your Blacktop rods with something better. Over 9000 it would be good to go with a quality forged rod. 

Crank ladders help reinforce the main caps. Tomei Crank Cab Ladders are one option. We decided we could make a design that was at least as good and cheaper so we did. Ours have all corners rounded off to eliminate stress risers that could lead to failure. 
http://www.matrixgarage.com/products/matrix-garage-crank-ladders
 

We have rod options from fairly affordable like the Crower Sportsman Rods to mid level such as the Crower BC625+ Rods and the Tomei rods all the way up to Carrillo rods that are used in Formula Atlantic.

Next we will talk about engine management. The path you choose will likely be influenced by what you are starting with and where you are going.

You can run a little boost on almost any ECU. How much depends on many things.
Before you even order turbo components you should have a wideband ready to install. It would be good to have it installed a while before boost so you can get familiar with how the car behaves in stock form.
Same if you are converting from GZE to GTE or even throwing on a bigger SC pulley.  

NA 4AGE ECU
The NA 4AGE can run a few lbs of boost with minimal adjustments. The AFM and FPR will add fuel to compensate for the added air. Once you get over 5-6 PSI these components alone don't have the ability to compensate enough. At this point you need to decide if you want to start throwing time and money at the ECU or if you should just step up to something better. Both have their advantages. Stepping up to something better means you don't waste time and money with something you are going to replace down the road. On the other hand starting out on the stock  ECU gives you time to get familiar with AFRs and basic tuning principles before needing to learn to tune aftermarket engine management. If you are going to have someone else tune for you then this is all irrelevant.
The 16V NA ecu has been taken up to 8 PSI on numerous occasions with minimal mods to fueling. On top of your wideband you will need to either play with different sized injectors or an adjustable FPR (fuel pressure regulator). One of these in conjunction with AFM tweaks should get you in the range of 8 PSI pretty easily. This doesn't soune like a lot but you are looking at roughly a 50% increase in power and in light little cars such as the AE86 and AW11 it makes a considerable difference. Many people would be quite happy here. Even better than an adjustable FPR is an FMU. These units increase fuel at a much higher rate as boost increases. 

A knock monitor at this point is highly recommended, especially since the NA ECU has no knock monitoring. A J&S safeguard would be even better but the one downside is that there are some full blown engine management systems for a similar price. That said, many engine management systems don't have knock monitoring and prevention systems as advanced as the J&S.
I put 8 PSI as the practical limit for the NA Largeport 4AGE ECU. It has been done and documented by others and should be relatively cheap and easy. I am confidant that with a combination of injectors, FMU, AFPR and possibly some other things you could run a lot more boost on it but at that point you will have spent as much time and money as many engine management options that will give you much more control.

Largeport 4AGZE ECU.
Many people think this is a logical step in the progression. Why not just install a GZE ECU and sensors or why not just buy a GZE longblock with ECU and wiring.
The US GZE ECU as well as the AFM ECUs from some other design markets have a huge handicap called low end leanout. Under 3500 RPM it caps the AFM signal. This means that if the motor sucks in more air than that under 3500 RPM the motor leans out because the ECU doesn't recognize the increase in airflow. You can add a gruntbox which uses the cold start injector to add more fuel and helps counter the low end leanout. For people who have the GZE ECU already in there car and are looking to run moderate boost this can be an acceptable solution but if you don't already have it in the car then the time and money to buy and install the ECU, sensors, and gruntbox will exceed the cost of some of the cheaper engine management solutions such as Megasquirt.

If you are starting with a 4AGZE and want to turbo it then up to stock GZE boost levels all you need to worry about is the turbo kit. If you want to run more than 8 PSI you need to decide if buying a gruntbox and working with what you have will be the best route or if you should upgrade to something with more control.

Smallport 4AGZE ECU
I don't have personal experience with this ECU.
I have heard it tends to have an issue with power falling off around 5500 RPM. One theory I have heard is it reacts to a false knock and pulls timing. You will also be limited on how much boost you can run before maxing out the MAP sensor. Like the LP ECU I wouldn't go very far out of my way to install this ECU. If you are swapping in a full SP GZE and are only hoping for near stock power levels then it may be the best choice. If you are looking for much over that then I would at least consider the other options before wiring it in.

Next are  the Map based ECUs including the Smallport ECU and the Silvertop and the Blacktop ECU. I am unfamiliar with running boost on any of these. I am confidant any of them could run a little boost but combined with the high compression of the motors they come with and the aggressive timing maps to make them sporty I am confidant it would take a wary and patient person to see what could be done with these setups without hurting the motor. Unless you already have the ECU wired into the car it would probably be the better choice to just wire in aftermarket engine management from the get go.

If you have a GZE ECU and wiring in the car you can run a BT or ST on GZE ECU and electronics.
This is actually a pretty good combination because the GZE ECU can only take about 8-9 PSI before leaning out and that's pushing a stock 20V pretty hard. I have been running a stock internaled blacktop on a GZE ECU with 7 PSI boost for 10k to 15k miles now.. On this setup I would consider wideband, knock monitor and pyrometer all mandatory. Even with the GZE ECU in the car this may or may not be a great way to go. If you don't already have a GZE ECU then I would say no way it's worth the time or money to go this route.

We have a number of affordable engine management options that will work great for many builds.

First I will talk about piggybacks. These can be an affordable solution and an acceptable one for some.
There are two primary types of piggyback. There are the ones that modify the signals going into the ECU. These do things like intercept the signal from things like TPS, MAP, AFM and the like and then modify the signal going to the ECU. These piggybacks should be avoided as they have many handicaps. They are also usless for the low end leanout the 4A-GZE has and can also change the ignition timing in unpredictable ways.
The other type intercepts the signal coming out of the ECU going to the injectors and the ignition. This gives you much more precise control over fuel and ignition and allows you to compensate for things like Low end leanout.
We carry the AEM F/IC If you are going to go piggyback this is the one to get. We can also get a Tweaked Performance plug and play adapter harness that will allow you to plug the F/IC into the factory harness and ECU and be ready to go.

http://matrixgarage.com/store/engine-management-tuning-tools

The DIYPNP is more for those comfortable with a soldering iron and circuit boards. This kit is not terribly challenging but can be daunting for people with very little experience. If you decide to take the leap however there is a ton of documentation and support.

The Megasquirt PNP is more expensive but comes completely assembled and literally ready to plug and play. You can get it with basemaps that should get you close but this will still require tuning once it's hooked up.

The AEM EMS4 is another affordable option. It contains all the features that most 4 cylinder tuners will need. It also comes with top teir software and support. We can also get a Tweaked Performance plug and play harness that allows you to plug this EMS straight into the stock harness.

We can also get LInk ECUs which are a great bang for the buck and we get great pricing on them. Email us for a quote. 
 

Choosing the turbo and surrounding components
This decision will come down to a lot of personal factors. You have to balance price, availability, long term availability, compatible components and your goals.
You might be stoked you found a great deal on some oddball turbo locally but if you design your system around that will you be able to find a manifold and downpipe to go with it or a replacement if it fails? Will you be able to upgrade to a bigger turbo if that one no longer suits your goals or move down to a smaller turbo if you decide it's too big and laggy?

One concern is product compatibility.
It is wise to see what manifolds are available before looking at turbos too much as that will likely influence your options.

Our shelf turbo manifold is available in DSM/MHI flange and T25/T28 flange. These are both great options since they have a number of options in size and price. We also offer a downpipe for these turbos.
We also have a Subaru TD04 13T in the shop that would allow us to make a mani and downpipe based off the shelf manifold for a similar price.
We will do custom manifolds for other turbos but would need the turbo to make sure it's positioned properly. Cost depends on many things like availability of flanges and difficulty of fabrication.
Before I made the made to order mani I am selling right now there were very few options especially for the transverse layout. Unless someone could do something custom themselves they would be limited to run a turbo that fit one of the very few manifolds available.

The final question is how much money do you want to spend and how leading edge do you want to be?
Used turbos can save you a lot of money and for most of us will suit our goals just fine. From now on I will refer to these as JYD or junk yard dogs.
Buying a used turbo always has it's risks. You could put it in and get 100k miles out of it or you could blow the bearings out the first time you step on it. Careful buying and preferably careful first hand inspection can greatly increase your odds. There are many sites, pages and threads on inspecting a turbo so I won't get into that here. Another option is to rebuild the turbo yourself or get it rebuilt. We have a turbo rebuilder and can also get rebuild kits for those who want to do it themselves.
The other option is to buy something nice and new off the shelf. This is very nice because the turbo is brand new and you can feel confidant it will last you a long time without trouble. Unfortunately many new turbos cost more than the cars we plan to stick them into. That can be a big pill to swallow.
If you decide to go with a new turbo we can help guide you in the right direction.
There are also journal bearing turbos and ball bearing turbos.  BB turbos are generally more expensive. They spin more freely and last a long time if treated right but if there is any issue you generally need to change out the whole bearing cartridge which is not cheap.  For most of us these turbos may not benefit us enough to be worth the extra expense. This is especially true for people looking to run moderate levels of boost on a budget. The BB turbo can shave milliseconds off spool time and the bigger turbo and longer it takes to spool the more effect it will have.  For anyone looking for less than 180 HP/liter and a turbo capable of producing that I don't feel that a BB turbo will be worth the  extra cost unless you are seriously racing the car or in other situations where milliseconds do actually matter.

Garrett makes many good turbos ranging from under 100 hp to over 2000 HP. They have been around and know their stuff when it comes to making, good, solid, reliable turbos therefore we have chosen to focus on carrying their line.
We only have a few turbos up on our site currently but can get anything they make.
http://matrixgarage.com/store/turbos-flanges-and-other-turbo-related-com...

We can also get a number of other turbos including Turbonetics, HKS, and others.

If you decide to go the route of a used turbo what one do you pick?
Of course you want one that will fit your current needs and goals. If you know exactly how you want your car to end up that may be enough. If you think you might want to grow at some point then you will want to find a line of turbos that use the same flanges so you don't have to change much if you get a bigger or smaller turbo. The most important part is the turbine side flanges for your manifold and downpipe. You spent good money for those parts and the last thing you want is to need to redesign your system and buy other parts or modify the ones you have.

Since the following talks a lot about MHI turbos I will go into brief detail on how they are sized and rated. There is a designation that starts with TD ie TD04, TD05, TD06. This tells you the size of the turbine wheel. Bigger number relates to a bigger wheel.
Next you will see something like 13T, 14B etc. This tells you the size of the compressor wheel.
The last thing you will hear people talking about is x cm turbine housing. This is the same thing as the A/R on other turbos. Instead of a ratio of the area to radius the mitsu turbos just refer to the area where the scroll begins. Bigger number means bigger surface area equaling a larger A/R.
There is much more info easily available on these turbos. One of the best places to start is here.

http://www.vfaq.com/index-main.html

Mitsubishi turbos have always been my go to. First off because I first got into boosted cars in the DSM scene and secondly because the turbos are very good units that fit the above requirements. There are quite a few turbos that all share the same flanges.
These range from some smaller TD04s like found on the 3000GT up through some TD06s capable of rather large numbers. There are also quite a lot of mix and match options easily available meaning you could play with your own combinations relatively easily. One downside of the DSMMHI flanged turbos is that most or the smaller options are getting older and harder to find. Turbos from the first and second gen eclipse, 3000GT and early Evos are all great turbos but also come off cars 18+ years old. This means they are going to get harder to find and generally with more time comes more wear. They are definitely not as plentiful as they were 10 years ago.
Unfortunately the Evo 4 reversed the direction of the turbo and went twin scroll. This means they are not compatible with any of the older turbos. They are also not compatible with much else smaller or bigger. The Evo 10 went back to the standard direction of rotation but still has the EVO twin scroll flange.
The DSM T25 is actually a Garrett turbo but has a DSM/MHI exhaust housing so it bolts up to the same mani and downpipe. It is a great turbo for those looking for quick spool and less than 250 hp. You do sacrifice a good bit of top end restricion for the quick spool but it Is often worth the trade. It is also the most common turbo I have seen run on 4As.

Subaru turbos have caught my eye recently because subies are so plentiful and I expect the turbos to be for some time. Many of them are also MHI turbos which I am fond of. Unfortunately they use a different turbine side. I am much less fond of the flange design on the subie turbos, especially the fact that they are only a three bolt flange to the mani. That said the subie TD04s are pretty common, pretty cheap and a pretty decent option for anyone looking for less than 250 HP. They are still a TD04 meaning if you found a healthy TD04 DSM turbine housing you could put it on a subie TD04 and run it on a DSM flanged mani.  There are nowhere near the OEM variations that the MHI flanged turbos have but there should be a decent flow of used after market turbos coming out of the Subaru community meaning you could probably find some interesting options available. If you can find a Subie flanged TD05 or TD06 housing it also means you could run many of the other MHI turbos.

Another nice option is the Garrett T25/T28 flanged turbos. These turbos are found on a number of cars and are still pretty plentiful and cheap. They are found on a number of nissans such as the 300 ZX and SR20s. Some are smaller and good for maybe 250 hp. Some may get you into the 300s.

Another very common flange style is the T3/T4.
I am completely unfamiliar with these turbos, their options, performance or OEM applications.
There is plenty of information on these turbos online.
I will also add any info anyone else has on these turbos or any other good options I have overlooked.

Aside from the manifold and the downpipe one of the bigger hurdles the average DIYer will face is the oil drain into the pan. If you are asking yourself if you can just use the oil cooler drain already in the pan trust me that you cannot. The diameter is too small and it will cause the oil to back up in the drain and flood the turbo. It is also on the low side. You want the drain as high as possible and definitely above the standing oil level. ½” is the absolute smallest line you will want to use. I highly recommend ¾” or 12 AN. This gives the oil a large passage and more area to flow through.
Most lines you find online are 1/2”. They will also require you to fabricate the bung in the pan.
I offer the service of modifying your pan with a 12 AN bung and building a steel braided 12 AN line to perfectly fit your turbo.
http://matrixgarage.com/products/4age-turbo-oil-drain-and-pan-modification
With the right measurements we can also build you an oil drain that will mate to your pan if you can modify it yourself.
We can get AN bungs and any other fabrication supplies you may need.

A couple other notes.
There needs to be a substantial drop on the oil drain line. The drain side is not pressurized and needs to flow freely into the pan. Oil that has passed through the turbo also becomes very frothy and won't drain as easily. Any transverse manifold for the 4A puts the turbo very low. You have to make sure there is enough angle on the drain line. This also means there isn't enough room for an adapter plate to run a different turbo on a different flange.
The reason you need such a big line and steep angle from the turbo is because turbos are designed to have oil lightly splash over the bearings and quickly drain. If any oil starts to build up in the CHRA it will start to leak past the seals and into the intake or exhaust. This will cause the car to smoke and won't take long to damage the seals. The oil drain line also needs to sit as high as possible on the oil pan. If it's submerged in oil it can back up the line.

Oil feed line. I can make an oil feed line for just about any application. I haven't found it to be worth advertising it much since there are so many large scale production feed lines out there much cheaper than I could offer them. Be wary though. Remember that this is one place where you don't want to cheap out or half ass things. Almost all AW11 turbo fires are related to oil dripping on the exhaust manifold. This is something you do not want to happen to you. The oil going into the turbo is under high pressure and sits right next to the header which can get extremely hot. On that note route the oil line as far away from the header as possible and don't be afraid to use some heat shielding if you are at all concerned.

Piping turbo to intake.
You want to use metal for your piping. I highly suggest something that cannot oxidize and flake material into the intake. For this reason I never use mild steel intake side. Stainless steel and aluminum works great and aluminum will generally be the cheaper, lighter and easier of the two.
I prefer not to cheap out on parts or materials but there are times I just can't justify the added expense. I have fabricated with, welded and run piping from Burns stainless as well as CX racing and no name ebay brands. The cheaper CX racing and no name piping welds just as well works just as well and usually comes with a nicer finish. I love to shop local, buy USA etc but this is one place where I buy the cheaper stuff. Since the no names are no names I can't speak for them in general but I have been happy with all piping I have gotten from Cxracing and ebay. I can now get kit's and individual pipes from CX racing.

One thing to remember is pre compressor the air is moving faster but after the compressor it is moving at about the same speed as when the motor was NA it's just much more dense. For that reason post compressor you don't need bigger piping when you run more boost.
One of the most important things though is having as few diameter changes as possible. For this reason 2.5” works pretty well for most 4A builds. There are a lot of piping kits available in that size, it's pretty close to but slightly larger than the diameter of the AFM and there are many intercooler options in 2.5” inlet and outlet.
If you use a JYD intercooler with smaller then you may be better off trying to match piping closer to that. I would never recommend going smaller than your AFM or throttlebody diameter.

Water lines
Not all turbos are water cooled but these days most are.
For the 16 valves it's easy to route the coolant line running to the throttlebody to the turbo. Make sure the water flows to the TB first and then to the turbo. The TB uses the coolant temp to control the idle speed. If you route the coolant from the turbo to the TB it will mess with your idle due to it changing the temp of the coolant.
20 valves don't have coolant running to the TB. On mine I teed into the heater line inlet and outlet. I don't have any proof on how well this flows but I do have about 15k miles on the used T25 I threw on this setup and it's still doing great.

Intercooler selection
This is another huge variable in the build. You can spend anywhere from nothing to thousands of dollars on your intercooling.
I would recommend running IC on any turbo build. If you want to run low boost this could be a cheap used OEM intercooler found at the junk yard or on ebay. If you want to run moderate to high boost I would put a little more thought into your system. Hot air is one of the greatest enemies of a boosted system. One of the hardest parts on MR cars is getting airflow. Front engine cars can do great by just sticking a big A2A IC in front of their radiator. To do that on a MR car would require 30 feet of piping.
There are some other options. Some have done roof mount ICs and or roof scoops to route the air into the IC. This does however increase the frontal area of the car and add drag. It also makes it pretty apparent that your car has been modified. This may be very undesirable in some areas or for some people. Others have moved their exhaust and put the IC under the trunk. This has it's own pros and cons. To actually get good flow you have to cut up your trunk and somewhere for the air to flow out of the trunk, it's still likely to see some heat from the exhaust. It's also very susceptible road debris and other things that could damage it. Many have gone water to air and then run the heat exchanger to the front of the car or in my case to each side of the car. This adds cost and complexity but is a very good way to get the efficiency of your IC as close as possible to that of front engine cars.
What intercooler you choose will have to be a balance of cost, effectiveness, space requirments and simplicity. If you are looking for a decent amount of power this isn't somewhere you want to cheap out. We are happy  to assist you in choosing the best intercooler for your build. 

Choosing a turbo that's right for you.
I'm not going to get into the details of all the variables that effect the performance of a turbo. There are entire websites, books, and forums devoted to that. I also respect that it can all be pretty overwhelming at first. Doing the research and finding those websites, books, threads and forums will help you out greatly. I also understand how it can be overwhelming trying to learn everything like compressor and turbine maps and what everything means.
I will recommend a couple websites that will help you fake it till you make it.
This will plot out your inputs on various turbo maps. One thing to beware of is that all those inputs have a huge effect on the result so the more you learn and understand the more accurate the calculator will become. It is still just a reference. It also only has compressor maps. It does not address the turbine map but few things do. It's rarely even possible to get a true turbine map.One of the best things you can do at that point is research the turbo on educated sites and find out what it has been proven capable of.
At that point start asking questions of your own.
http://www.squirrelpf.com/turbocalc/

This page will help you calculate intake temps.
http://www.stealth316.com/2-turbotemp.htm
And this page helps understand why and also how it pertains to the compressor map.
http://www.stealth316.com/2-adiabat1.htm
There is also a ton of other good information on that forum.

Garrett also has their own boost advisor. http://www.turbobygarrett.com/turbobygarrett/boostadviser

If you want any input or help choosing your turbo we are happy to help.