I dont have an s2000 but if you understand the concept of turbo sizing in relation to engine sizing then it will help you make you decisions...

If you want quicker spool, more driveability, less top end power go for a smaller turbo and vice versa.

From what i would say you would need something around a gt30 series mark. Being the 2.0 it would push that fairly well and the gt30 series has alot of potential for some nice power.

But if you just wanna add on some psi 8-10 with a nice low end power, low spool etc then go for the gt28 series...

or if you want MASSIVE HP and you just wanna be a DYNO Junky look into the Gt35 series.

Also you have to look at exhaust sizing, turbo manifold (short ram, long ram, logg) External wastgate or internal, intercooler size, piping size.

All these things are changed by the size of the turbo, PSI you are running, how much power you want and how quick you want the turbo's to spool or how much spool time u would wanna give away for a greater HP top end.