Stock r34 Gt-t or else something small like a HKS gt2510 or gt2530.
But remeber when putting a turbo on an n/a engine you should do the following
1) buy turbo injectors
2) buy some-sort of I/C (can be stock or aftermarket fmic)
3) Turbo Fuel Pump (or aftermarket one)
4) Somesort of ecu (safc2 is fine or pfc is you really want to spend the money)
This 4 mods will help make the na to turbo engine much more safer. i have a friend in perth who turbo his n/a r33 using stock r33 turbo and has made just around 270rwhp (he is running abit of boost, fmic, turbo injectors, safc2, full-exhuast, air filter)
hope that helps
Michael
p.s i just remember it is also a good idea to upgrade your brakes from n/a to turbo brakes (even if its just the front)