If you consider a turbo + supporting mods + tune to be "bolt-ons," then the power potential depends on how much the engine can handle before it needs a rebuild... ballpark, maybe 200-350 whp, unless you go with a stupidly large turbo. It also depends how well the car was maintained over the years and how you drive it.
General problems are:
1) Installing an atmospheric-venting bov = car will run poorly.
2) Intakes = generally short rams are better than cold air intakes... do research before buying.
3) Stock WRX 5MTs are known to be weak when you bump up the power significantly, especially with aftermarket clutches. The general rule is: stock transmission + stock clutch.
4) Bad tires. They need to be the exact same brand, model, and size. Also, they're best within 1/32" to 2/32" of each other or you risk damaging your differentials.
5) Rust.
6) Buying parts without doing appropriate research and not getting tuned immediately after installing them.
That's just a general list to get you started. If you take care of them, they can last well over 100k, even when modified. Worst case, you might have to shell out a few grand for a trans or engine rebuild, but they're pretty solid cars otherwise.
There are several decent threads about what to look for when buying a used WRX or STi. Since you obviously plan to modify the car, you might also want to check out this site for some good Subaru-specific info:
Unabomber's Manifesto - NASIOC
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