As to the HP out put of the motors: the USDM 2.0 WRX puts out 227 HP.
When you are talking about the 2.5 turbo, I assume you are talking about an USDM STi motor ? That's 300 HP.
If you are talking about the '06 2.5L WRX motor, that one puts out 230 HP. (But I'd be surprised if you already got your hands on one of those
)
You want 400HP ? Have you ever driven a high powered car (or bug).
I'm not saying I have, but before you aim for goals like that, I'd say give it a try first. You might find that a 227HP motor is plenty.
Like Ryan (73Notch) says, keep it simple, you can always upgrade later.
-sorry for the long post
Rob.