STi's from MY02 onwards are great cars. The 2002 is in my opinion the best value of the lot thanks to the questionable styling (which actually grows on you with time)
Things to look for.
1/ Make sure all the ID tags match the car, ie; must show EJ207 as the engine code and those tags must match the bodyshell.
2/ pull the boot carpet up and make sure the car is the same colour in there as the rest of the car
3/ Inspect the seat, seatbelt, door trims, pedal rubbers and steering wheel for wear that doesn't match the milage on the car. (quite common to be changed for lower km on these cars)
4/ Wheel bearing failure is quite common due to large brakes and relatively small wheel bearings.
5/ Centre diff tightening up due to mismatched tyre sizes and brands.
6/ Slow synchros in the gearbox when warmed up, particularly 4th gear.
7/ Cracked radiator top tank near the radiator hose. (not a big deal to fix unless the car has been overheated due to the fault)