You will hate going from that neat as S15 of yours to a 18+ y/o GTR, much rougher, older, ect. Trust me on this, I had the exact same idea , cept I wanted to mod the shit outta shit! Even if you find a super stock clean one (which will be hard and VERY expenisve) there is always stuff that just does not last 18+ years and will break.
I would be looking at 33GTR's as they are a total bargain at the moment.
I almost purchased this one, but then decided to go back to uni. Uni + GTR = no money for food!
http://www.j-spec.com.au/list/index.php?ID=11540