Here are your choices listed in ease of swap to difficultS13 SR20DET RedtopS13 SR20DET Blacktop (newer but same as redtop)S14 SR20DET Blacktop (this version have VTC and Ballbearing T28 Turbo)S15 SR20DET Blacktop (has VTC and Ballbearing and is more difficult to get into S13)then there are the skyline engines.... which I'll leave out.
S13 and S14 pretty much just drop in but you want to buy a front clip or a complete kit such as those offered by
http://www.heavythrottle.com. They are a little priceir then other sites but their service is spectacular...
The S13 red and black tops, people say, will give you more HP if that's what you are after while the S14 and S15 black tops give you more HP from the get go but if you were to put some serious hp down on the pavement, it won't push numbers like the S13 red/black tops. Personally I'd go with the S14 Black top but that's just me. if you go with a SR20DET, no matter which one you choose, you won't regret it because no matter what you are into, drift or drag, the SR20 can do both quite well and very competitively if you put some money into it that is, just like any other motor.
Want raw gut wrenching, enough G's to make you pass out, acceleration... then you can look into the skyline arena. The RB's will probably scare the living death out of you if you decide to get one of those... atleast it would to me. However, it is much more difficult to drop in a RB. I've done 5 SR swaps and if me and a buddy are to do one, in about 6 hours (more if we spend most of the time looking for tools like we always do lol) can rip out the old motor and drop in the SR and have it running just to give you an example of how simple the swap is.