Like most of the people in this thread, I would suggest getting an evo. Price for a used evo vs a used STi is a bit higher but you can make 400whp on the stock turbo in a evo for fairly cheap, compared to 400 whp in a Wrx or STi.
Guy I know did all the assembly of his STi motor, and got a killer deal on mostly all the parts, and machining ect. Let's just say on a fully built, sleeved 06 2.5 STi block with forged internals ect and built 08+ STi heads with 280 cams he maxed out a dw 300lph at 488whp.. on e85
This is after pouring 8-10k in the build.
His stock turbo evo 9 on e85 with no more than 3k in parts and tuning is making 400whp.
If I was going to build a super fast car, it would be an evo due to the pricing, and temperament of the 4g63.. Big turbo no tune.... No problem..
STi with a intake and no tune... Motor is unhappy.