Build quality and signs of bodywork damage. Has it been looked after really. There's no particular thing to look for but options might affect your decision - engine type and size, wheel design. If you want Coffin Spokes then 5 stud is necessary and conversion isn't cheap. Group 4 cars more expensive than Stradales generally but you may or may not like the paint scheme!

Treat it as any normal car and check the paperwork matches, was it IVA'd and is the V5 description correct.