hmm... why got your heart set on the WRX motors? They're other types of converisons, ones that may not require as much $$$ down the road and still offer you a good deal of reliable power.
And it is true about the custom work needed. But things like throttle linkage shouldn't be too hard, you should be able to run a cable from the end of the chassis tube, I think that CB sells one for they're single carb turbo kits, it's got a fancy machined put that just rides in the stock tube, this way you don't need a bracket. Routing radiator lines shouldn't be that bad as long as you take your time with it. I think the biggest problem would be trying to wire the ECU harness in.
The way I see it conversions are for those willing to trade one set of problems for another. Type I's or even type Iv's for that matter are easy to get in the car; the problems that arise are they're reliablity. Type I's are designed with about 4times less hp than what many people are expecting out of them. They are not full flowed, so you have to worry about filtering the oil and probably cooling it a bit better. Type IV will take care of this problem, and it will cost you. You'll spend around 600 bucks on a good exhaust system, not to mention the cooling system will run you a bit if you go upright. But the key benefit here is that it isn't "new territory" You have people that have been working on these motors and the parts are easily accessable, (though the type IV parts will run you a bit more). And be carfull of the lifter problems people have been having.
One of Jake's 2270 type IV would be a great way to go. But, the cost may be close to what you would pay for the pieces to do the conversion. One of Jakes motors would be great don't get me wrong, they are strong, powerful, and reliable.
With the converion if you pick the right motor you should be able to get 100 thousand miles out of it, with out tune up or adjustment, and it would be fuel efficient which is a good plus. The serious problems here are if you ever have to rebuild the motor. But I beleive now FLAPS can actually get rebuild motors for not too much. Or if you rebuild it; finding a place that handles your motor. But if you choose a newer toyota, ford, vw, or mazda this shouldn't be a huge problem.
Check this out
http://frost.bbboy.net/vwengineconversions
Frankly what I would do is find a performance forum like this one for the engine your interested in, this way you can talk to guys that know about your engine, this way you can find out the costs of a rebuild and see if the problems with that motor are somthing you can deal with. This will take time though. Search the forum. I really like what the mazda rotarys have to offer but... this is not to say it doesn't have its problems.
Every motor will have problems, its a matter of which are you willing to deal with. Find out what hp range you are looking at then see what fits the bill. And price it out, everything. See what it takes to make a type I with your power range. May be a 1776 turbo would be best. Get it full flowed and balanced well. Or a 1.8 n/a out of a jetta might be better.
Rip