Before you even start it, pull the dip stick and check the oil. It should look fairly new. Black sludge means they never changed the oil; cloudy or milky oil means that the head gasket is about to blow. I looked at an SUV once, and refused to buy it due to milk oil. The girl selling it got mad, told me it ran fine...and then the head gasket blew ten miles down the road after she left.
Ask to see maintenance records. Ask about the timing belt -that is a big deal if it has not been changed on schedule.
What is your budget? What kind of car do you want? Do you want to go fast or be reliable?