I have gotten to the point with my STi that I really don't know if I want to take it any further than where it is. (stage 2, headers, suspension upgrades) It's a blast to drive on the street, and starts and drives every single day without me giving it a second thought. I am seriously considering looking for a 1G GSX or TSI to build into a race car. After seeing a 1G bought for $400 out of a junkyard run 11.2 on just a FPGreen (gutted interior/slicks), I was in awe. Running the STI that fast I'm looking at a GT30 kit at least, and that's hoping that the ring lands hold up. Ok, so I'm babbling. Buy the WRX for a daily driver I'd say. The newest GSX you'll find is 6 years old and probably fast approaching 100k miles. Like Luda said though, if you're looking for a project an older DSM is going to be much better bang for the buck.
FP Green'd 04 STi