Engine
Although STis have uprated engine internals they still need to be cared after in the same manner as the WRX motors. Oil changes should be carried out every 5,000 miles for Classics or 7,000 miles for newage, although more regular changes are not a bad thing, new spark plugs, brake fluid, transmission and diff oil should be changed every couple of years or 24,000 miles and the cambelt should be changed every 45,000 miles.
Listen for knocking coming from the engine as this could indicate piston slap or bottom end damage and the turbo should run quite quietly if the cat is still in place. And last but not least, smoke free. Turbos must also be allowed to cool down after hard driving - ask the seller what he does for the shutdown procedure. Watch for a fluctuating idle which could be a symptom of a blown mass air flow sensor or sticking wastegate solenoids. If a boost gauge has been fitted, check that the boost pressure is not exceeding 17–18psi.
Exhaust
The standard exhausts are fairly long lasting but most owners will have probably have replaced the original exhaust with a stainless steel aftermarket performance exhaust system, due to their performance and their very reasonable prices. Hayward and Scott to name just one. If the cat is removed on the Impreza you are looking at buying make sure to get the old cat from the seller, if it’s not available budget this into the price, as you will need one come MOT time.
Transmission
A powerful car with 4wd is always going to punish the gearbox especially so if it has been modified. It’s not uncommon for a clutch to last as little as 40,000 miles. Clutch judder is common on Classic’s when the car is cold. The Subaru gearbox is extremely tough but when the gearbox is warm do the usual checks to ensure that all gears engage easily with no crunching, listen out for any whining at all whether it be from the gearbox itself or from the diffs. Popping out of gear is almost unheard of, but accelerate hard from low revs in each of the gears to make absolutely sure.
Brakes
The OE brakes and pads are not amazing and most owners uprate the pads, discs, lines and fluid to uprated items. DOT 5.1 fluid is common, especially if your going to take the car on track. As with any car brake hard from speed to ensure the car pulls up straight and smooth with no judder. Rear brakes can seize if a car is left standing.
Bodywork
Accident damage is a real possibility on these cars, vulnerable areas include the front end, front and rear wings. Check the underbody thoroughly for rust as a lot of car will not have been under sealed. The bonnet is also vulnerable to stone chips, budget the repair of this into the purchase price.
Interior
The electrics are general sound although the cheap interior plastics can look tatty over time. The newage cars have better interiors however the seats become less ‘buckety’.
Suspension
Worn anti roll bar bushes can be diagnosed by knocking coming front the front suspension. It has been said that the best suspension available for the STi is it the original STi set up. So replacing does necessarily mean having to go down the performance suspension route although this may work out to be the cheapest option. If buying the older Classic shapes check the rear strut's more so if it's a UK car as they seem to rot/rust at the top around the bolts.
Feel free to add anything that might have been missed I will updated the post :)