R32 gtst would be a smart buy. Go turbo if you can afford it, or you'll regret it once you want power, and of course sucking noises.
I'm going to rely on my memory and contradict everyone here: R33 GTS25's are eligible under the SEVS scheme in that they exceed the power to weight minimum. However, the market may consider importing one not worth the import and compliance costs, implying that R33 GTS25's would not be imported under the SEVS scheme, althought they are potentially eligible.
Non-turbo silvia's and RB20 R32's do not meet power to weight minimum, from memory.
-Greg