Do a compression test...If all of your numbers are within 10-15% of each other and still holding good compression then boost it...dont go crazy with the turbo. Nothing wrong with a smaller turbo that spools faster. Forget about the boost pressure since 12 psi is just a unit of measure. You need to have an understanding of compressor maps and such. 12 psi on your avg. t3/t4 is way different that 12 psi on a 14g dsm turbo. Do some reserach on people that have built similar set-ups and go from there. IMHO small t3/t4 or bigger t3 will be perfect for ya. run 7-10psi and get a tune. dont skimp on fuel parts...if you have the fuel and a great tune the car will last. I ran a drag gen III on a stock gsr swap for years. I sold it 4 or 5 years ago and the car its on is still running like new.....its all in the tune. Buy quality parts and get a good tune and you will be fine as long as the motor is in good condition. my gsr swap and the gsr the turbo kit is on now both had well into the 100k's on mileage and both are/were beat on regularly.