STi cars were not available before 2004 in the US. I own a black WRX and would buy anything except black. Subaru paint is horrific.
The car responds better to fueling and timing than directly to boost, and is best modified from the wheels up. You'll almost certainly need to replace whatever summers are on there, and if you are not sold the car with winters if your climate has snow or is very cold I'd look into that first. Then I'd check that the pads are OK - the stock brakes are more than adequate but the parts are co$tly. The rear dampers can get thumpy, it's a common fault or trait (it's a fault if you're paying, it's a trait if you ask the dealer to pay sometimes...). Leave the motor alone until you can read the sticky threads on each of the fora here and/or you've sorted the car first. Enjoy, welcome.