Why 500awhp? Have you ever ridden in a 300awhp car? To get to 300awhp (which is still PLENTY fast for the street) all you need is a nice exhaust set up with some ported/polished manifold elbows, perhaps a short-ram intake, 255 fuel pump and a good tune. If you want 400awhp you will be looking at a 49-52lb/min turbo and highly recommended to swap out at least the pistons, massive injectors, FMIC and EWG. 500awhp is in no way easy if you want your motor to last. I'd recommend a GT35R .82A/R (minimal), fully built bottom end, some nice ported heads, cams and one hell of a valvtrain so you actually have a powerband you can rev to. In other words- $$$$$!
If you just purchased you're 04 STi, I'd recommend getting used to it stock then slowly add power. A strong "stage 2" setup is a very potent combination for the street and reliable. Do not forget suspension and brakes too!