hmm if you want high 12s and a clean car with $15,000 i'd get like a 95 s14, get your sr20 and do a bottom end build-up, tubular manifold, 2.5" i/c piping, fmic, more efficient turbo, 3" exhaust, then spend the rest on electronics and making the car look nice. with a built bottom end sr20 you'd be more than capable of doing high 12s probably without too much tuning, as long as you got a turbo matched with what you want to do. and at the same time you could have a nice looking, good riding, classy car.

try srswap.com (like goth said)
www.nissaninfiniticlub.com
and look up afterdark motorsports (i think thats what its called)

there are a few sr20 gurus on the board that probably wouldnt mind helping if you got some real technical questions.

edit: bottom end build might be out of your budget unless you get your s14 for like $3,000 and the sr20 will hold up past high 12s anyways. skip the buildup