also see if the engine/ecu setup you are getting has that infamous security system that subaru engineered into some of the JDM cars, but did not make it over to the US market - namely a key with a transmitter that works with the ecu. without this key, the ecu is almost as good as useless.
Assuming stock RS - this is your quasi-definitive list of needs:
(I assume here that you are getting a STi engine)
- Turbo cross member (failing that, any wrx cross member will work)
- Complete engine (turbo/IC/radiator - oh yeah, radiator supports since they're different!), wiring harness, ecu
- tranny + matching diff (if possible) which opens up the options which ill list in a moment
- half-shafts and axels.
- matching 4 pot fronts and 2 pot rear (which usually means you have a R180 rear diff, as opposed to the R160 we have) brakes
- variable centre diff - which you'll need the controller (separate), wiring, and matching switch
- other suspension bits that may or may not come with what you buy
things to look out for
- avoid the twin turbo ej20 engines - as the 2ndary turbo will give you clearance issues - not that it's not doable, but it's a hell of a lot more work
- get compression/leak down test numbers, along with mileage on setup if possible
- warrenty if possible (i.e. not DOA and so on)
Things i recommend you do while you're partially tearing the car apart and trying to put the new one in
- Clean the IC with either gasoline or other cleaner - as it's probably dirty
- do the tune up/prep work while it's out if you have the room/tools to do it (i.e. get the engine running before you stuff it into the car)
- fix any suspension stuff while the engine/trans is out - it's just a lot easier that way.
oh yeah, did i mention that you should also keep about another grand in CASH handy - in case you need to score things that you didn't realize you need, or just found out doesn't work.
even with the assistance of a very large hammer.
I've learned this from doing a couple of swaps - into imprezas and foresters. I still think the impreza is the easiest and most flexible out of the lot to do.
One note: You're not exactly limited to the STi engines out of an impreza. One intriguing prospect that i recommend doing (assuming you have the time/resources) is to find a Forester STi engine with the matching diff and so on. In all reality, you'll never find a place to really go as fast as you'd like. So why get some more torque and have a lot more fun in between the gears instead? The essential difference between the Forester and Impreza STi engines are the cams and the ecu mapping to compensate for those things. Imprezas have higher hp/lesser tq, whilst the Forester has higher tq/lower hp to account for the extra mass. But since you don't have the extra mass, you'll be amazed at the extra performance.
Yes, this torque over hp route is not conventional - but it's more fun based upon my rally experience.