i'd suggest you look for a pair of nice juicy CAMS to pair you along towards the 400-500hp range, at the same time be prepared for some lag, due to small displacement you have in comparison to a 2.0litre or 2.2litre.
JUN makes some awesome cams for the 1.8 GSR.
As for the turbo,
if it's gonna be a street fighter, then opt for a GT3037R with 0.82 turbine, it'll give you a nice 450 ATWhp...IF tuned and paired well with your other hardware.
the Bullseye DSM256 is a good drop in for your stock manifold, but if i recall correctly the GSR ports are rather small and might suffer restrictions. Just to take note....
If budget permits, do look at a nice and proper flowing intercooler to chill the air. Don't opt for those China made ones.. since you wanna do, then better do it proper.