350-400kw atw?
Prepare to spend big (c.10-15k) and put up with lots of little issues- especially if the car isn't in tip-top condition. The difference in cost between 250rwkw and 350rwkw is big.
For that power you'll need a forged bottom end, head work (cams, port/polish, valves+springs), fat ass laggy turbo, reworked fuelling system, Power FC, twin/triple plate clutch etc...
I could go on and list the other things, and some of the problems that might arise from working the car so hard (such as maintaining traction)
If you're prepared, patient, well planned, and have the cash you can build an awesome machine. Do some searches in forced induction and drivetrain sections too.
Good luck