My thoughts,
Do other mods first as Gus said - do suspension and drivetrain first.
Do a few driver days first, learn how your car goes first, learn how it handles etc
If you just have an exhaust as only mod, I'd look at doing a fmic, boost controller and a retune. this would give you a slight power increase, could even go as max as 160rwkw.
That and some suspension mods and a 1 way diff at a minimum would be good fun!!
I had my line for 2.5 years before I upgraded my turbo!
But if you have your heart set on a bigger turbo - basic upgrade would be an r33 (not sure if r34's bolt up?)
Cheers,
Chris