Just buy a 2005+ Sti for the better bolt pattern and stronger hubs. The cost diff between the 04-05 is not much. They can be snagged for $18k-$25k depending on mods and mileage. The 06-07 are still upwards $25k easy. Knowing you, I know you dont want a 08 STi due to the hatch. Do your homework and get as much info on the car (if used) as you can.

04-05 = Blob Eye
06-07 = Hawk Eye
08-09 = Hatch

In regards to power, you can hit the 400awhp mark quite easy. Exhaust, FMIC, Tune, larger turbo.......