Gonna slum it with a free Z31? What happened to the S2000/EVO/FD plans?

Nissan already gave you the best engine swaps for that car from the factory. You could start with the Turbo model engine that came in that car, which makes as much power as an SR (210ish) and drops right in. VG30ET is the name and it should be dirt cheap. They're basically bulletproof and come with a strong trans. Slap on a bigger T3 and go. Also, Nissan sold the same car with an RB20DET (series 1) in other markets. Google 200ZR. You can get the relevant parts and swap in any flavor of RB motor.

If you really have and EVO/S2000 budget, you can probably go over to ZFever and talk over your options. They've been working on those cars forever and will put whatever motor in there that you want.