the closer the car is to stock, the better.
mods = less reliability.
basically find an STi with the lowest amount of miles you can for your budget and test drive it to make sure the turbo spools well and holds boost, check for codes, make sure clutch is not soft, check for play in shifter, then take it to mechanic for compression test and other tests the mechanic recommends. Its just like buying any other car.

a blowoff valve is a telltale sign that the car had an idiot as previous owner.

*braces for flames*