One piece of advice I can give you, from first-hand experience: never buy a car that was warmed up when you test-drove it.
Folks will let a car or truck warm up and then shut it off just before the prospective purchaser arrives. That's usually because there's an issue somewhere that is audible when the car is first started, or that it has an issue with starting: either way, a vehicle already warmed up is going to rob you of some of the details you need to make an informed decision.