Now for the other crucial step that you must take: Get the vehicle inspected by a qualified mechanic. Please people. I hate to sound preachy, but over the years I have gotten hundreds of calls and emails from consumers who were stuck with cars too dangerous to drive because they failed to take this basic precaution.
So, please, take the used car to a mechanic before you buy it. It might sound like a pain, but it's really not. For one thing, there are mobile mechanics who can bring their diagnostic tools right to the car. Also, AAA-approved repair shops offer AAA members a free 24-point inspection of their vehicle.
You can get this same inspection for a vehicle you are considering buying. If the dealer or owner won't allow you to get the vehicle inspected by a mechanic, I think you know what to do. Run -- don't walk -- away from the deal.