first thing to check on any GC subie is condition of bodywork around rear wheel wells. most or all will have bubbling where the rear quarter panel and plastic bumper cover come together; this is a well known and easy fix for any competent body shop. but if the curved metal around wheel arch and bottom of door frame is rotted, that is terminal cancer. run away.
likewise, inspect rear subframe mounts. if rotted, run away faster.
oil leaks around r main seal and cam covers, crunchy synchros and vile smells infect many well-worn OBSes out there, beware. many have been flogged in daily driving for years, carrying barfing kids, shedding canines and all manner of hard-edged cargo. many have serious deferred maintenance needs.
otherwise they're pretty solid. many WRX parts--mechanical, suspension and interior--are direct fit, so you can easily create a very distinctive ride.