400 crank HP is doable with a standard 2.5L 944 turbo engine.
Once you aim past that is where things get expensive.
Easiest way to get there is buying this kit:
http://www.lindseyracing.com/LR/Parts/951PACK340.html
Now some folks have said the "340 rwhp" number is a bit inflated but it'll get you pretty close to 400 true crank HP.
It'd be hard to do a decent LS swap for the ~$4k the kit above costs.