Ford put a bewildering collection of engines and specs into Sierras over the years, so check which one you’re getting. Pinto units are reliable and long-lasting, given regular oil and cambelt changes.
Earlier gearboxes can be notchy and jump out of gear, whereas post-1989 cars have a stronger ‘box. Watch out for worn MacPherson struts, coil springs, and front suspension bushes.
Make sure that your Sierra isn’t about to be sacrificed to rust, because the sills, wheelarches and boot floor can all suffer terminal tin worm. And replacement plastic mouldings are getting rare.