Hi there - first thing to consider is what you consider doing with the car ; fast road use and occasional track use and gearbox should be fine, but if you're drag racing then that will hammer the 'box (tho mine lasted 4 years of drag starts before going bang).
Also be aware that as you put more power into the car, then the wallet has to come out for other things - brakes etc etc
400bhp or thereabouts is obtainable without spending insane amounts of cash - but before you start is the engine in good condition? Anyway, my spec is as follows and I'm running 380bhp & 325 lb/ft of torque at 1.6 bar - and the turbo has more to give...
Completely standard engine (top and bottom end), done 150,000km.
Quaife Gearbox with Helix Organic Clutch and Lightened Flywheel
GT2871R Turbo
Janspeed Turbo back 3" Exhaust incl turbo elbow.
Tubular Manifold (made in the UK, not a $hitty ebay one from china!).
550 cc Injectors
Evo 6 FMIC
Apex'i Type Air Filter
Dastek Unichip
GReddy Profec B Boost Controller
Blitz Turbo Timer
Brembo 4 pots with 305mm plain discs (from a Seat Cupra R - brackets from Godspeed)
GAB Suspension
Nismo Strut Brace
The engine itself should be fine, but if in doubt have it checked out by a friendly mechanic - compression tested etc etc. If you need any more info just PM me.
Cheers,
donpulsar