Most common issue is heads and head gasket. A couple of simple test are to get it running, warm and check the pressure on the main hose from the radiator. If it is rock hard it means their is an engine leak that is presurizing the cooling system. Also, check the coolant expansion tank. If it empty, you likely have a problem. If there is fluid in it, take it for a nice drive and check the level to make sure it is the same as when you started.
Timing belt. Regardless of mileage most are so old they should be or should have been replaced. You can't easily check, but you can ask for receipts. Replacing this can be an easy 1.5-4k cost, depending on how much you replace.
Crawl under the front passenger wheel well with a flash light and look up at the turbo, alternator, oil filter area. Any sign of wet oil can be one of many things. The most common of which is leaking at the oil intake/outtake on the alternator. This is a common weak point. Cheap fix, but a bitch due to the location.
Check the grease points. This is usually a pretty solid indicator if the vehicle has seen much maintenance because this is easy to do.
Check to see if the radiator is original or not. This is another thing that, regardless of age, should have been replace on all older vehicles and is an indicator of how well the vehicle has been maintained.
I could go on an on, but these are some basics. Keep in mind that people tend to think these are all in great shape because of the low mileage. Plus everyone says they have maintained them. But in reality, actual mileage and just time have significant impacts on things like belts, hoses and gaskets. And to most people maintaining a vehicle means changing the oil and dealing with any issues that have popped up. On older vehicles preventive maintenance is just as, if not more, important.
