Hi I have a 2005 WRX, so check rear wings and rear sub frame for rust. The wings could rust from inner wing out. Check for any whines from gearbox ( it's the week point of the car) anything not quite right with gearbox walk away. Just cost me nearly £1k to have mine reconditioned. Engine is pretty much bulletproof, but check service history. Especially for oil changes. So change oil every 3k miles, but every 6k miles is fine, depending on how it's been driven. Then just check for usual signs of accident damage. Maybe an hpi check for piece of mind. If any mods have been done check who done, ante they good? Other than that the rest is normal things to check for. All imho.