Carby issues (high idle once warm, hard starting) and rust. Check the seams on the floors, wheel arches (on the inside), around the windscreen hinges and above the front door hinges on the font 1/4 panel.
You can also check for cracks in the panels around the windscreen wipers and the tops on the doors but I'd expect every 1980s Sierra to have those cracks by now.
Those are the main issues they have. It's a 29 year old car so there could be any number of things wrong with it, it does look super clean though. Good luck.
