1st off...that ebay EJ20G is EXTREAMLY overpriced.
The EJ20G is a great way to go fast on a budget (im putting one in my car in the next week of 2).
The wiring isnt to diff. if you know what your doing. you will need to splice in the EJ20G wiring into the stock wiring harness. What you do is use the EJ20G harness (from ECU to engine harness) but use all the stock internal harness, like for your lights, blinkers, ext. if you know what your doing, the wiring will take you anywhere from 4-7hrs.if its your 1st time, it might take you 2-3weeks.
Also another thing to consider. The stock tranny WILL NOT LAST. it all depends on how you drive, but i would not rely on it for longer then 3months at most. what would happend is eather you would break your gears from launching, shifting to fast, or break your shift fork, and no matter what your syncroes will go out also.
as far as engine managment goes, pritty much the only real option is the PowerFC.
if you wana be cheap go with the EJ20G
If you got some more money go with a EJ20K, or EJ205
If you got more money then you know what to do with buy a USDM or JDM V7/V8
and the best thing i can tell you is BUY A FRONT CLIP! i ended up buying everything seperate, and it cost me more then a clip would have cost me. You will need X member, and couple of other things that are eather hard to find or expensive.
also look around this thread and you will get more info. The EJ20G doesnt get much support from allot of people becoue its older technology and not as fast as the newer engines, but for the money you cant get a better deal.