theres a massive difference between 250rwk and 350rwkw so you should think carefully before deciding, the cost difference is around 10k.
250rwkw it would go for hks gt-rs or garret equiv, z32 afm, 550cc inj, powerfc, heavy duty clutch, tyres, suspension work, cold air intake, intercooler, boost controller, 3.5" split dump zorst and cat back
for 350rwkw it beyond the safe internal limit for rb25
so youll need to open it up to safely make that power.
ie: cams, pistons, injectors
you need a larger turbo (more and more lag)
with forged pistons, cams most likely
bigger clutch, better suspension work
and so on
as the power graph increases linerally the cost graph increases exponentially.