Rust is a problem and repair sections are are generally not available. Pay particular attention to suspension mounts on all 4 corners, around the mid-mudflap mountings, the “tunnels” the driveshafts pass through and the box section the rear shocks mount to. All of these points are MOT failures. Floors themselves are relatively flat with inboard sills so suffer less than most other cars of the era. Cosmetically the whole lower quarter of the car is vulnerable but check door bottoms inside and out, front valance, bottoms of the rear quarter panels, scuttle panel in the corners of the screen and lower sections of the wheelarch lips (under the trim strip).
Some mechanical parts are scarce so check everything works as it should. The cooling system is under significant load on the turbocharged cars and the waterpump is a known weak spot on the B series engine and requires difficult to find parts and specialist tools. Check the car runs in or just below the middle of the gauge, that the cooling fan cuts in if the gauge goes much higher than the midway point and that there is no evidence of water in the oil or oil in the water.
Check that oil changes have been done every 5000 miles with quality synthetic oil, this is vital on the Turbocharged cars. Tuning and setup is critical to to safeguard the engine and give the best performance so check for evidence of regular servicing throughout. The k-jet injection system is relatively simple and robust and shares many components with other cars of the era (including early Golf GTi's, Porsche and Mercedes models) A k-jet system in good order should need hardly any adjustment as the setup doesn't tend to "drift" unless there is a faulty component. Cold and Warm idle should be smooth and there should be no miss-firing or flatspots. Gearboxes can be a bit clunky when very cold but should change smoothly when warm. crashing syncros or jumping out of gear may indicate a worn box. My own car has a noisy input shaft (slight clattering in neutral and a mini like whine in the lower gears but I have chosen not to worry about it too much as it hasn't got any worse in thousands of miles)
Turbo needs to be given time to cool at idle speed after being driven hard or at prolonged speed otherwise oil is baked onto the bearing surfaces causing early failure. Quiz the owner or pay attention during the test drive to make sure they have been doing this. Any turbo that has been rebuilt at the Garrett factory should carry a small plaque on the compressor body.