250k i guess-- doesn't matter too much if it's been maintained well and the still seems to run fine. As far as the engine goes, around 150k-200k I'd ask if the head gasket, timing belt, fluid pumps, radiator, & plugs have been replaced, and ask if they've done a compression check for each cylinder. Visually inspecting all the hoses is a good idea; cracked and crusty is obviously bad. If they have full repair/maintenance documentation then that is a huge plus. I personally tend to be more worried about transmission/clutch wear when looking for anything over even 100k lol
what is the car you're looking at?