If you plan on doing so many mods to your WRX. That every major component is going to be replaced with a new high performance part. Why not start with a Impreza 2.5i?

Example: Closed Deck motor, bigger turbo, coil over, tiers, rims, trans, radiator, ect...

I really would like to make 500 to 600whp

My thought behind this. The 2015 WRX SIT is about 20k. The Impreza is about 12k. All of the parts should be about 12k + labor for some things I can't do myself.