The hardest part buying a used car is, especially if it is one you like, do not get emotional or attached. Do a thourogh test drive, at least long enough to warm up the engine, and test out all systems on the car. Some problem dont show up untill car is nice and warm, and has been driven a while. If you can, get the car onto the highway and see how it drives. Chance are that at somepoint during your inspection you are going the find a somthing wrong, you just need to decide if it is a deal breaker, or are willing to repair regardless off the sale. Watch out if the dealer limits your test drive. Offer to put a couple buck of gas in the tank if they are worried about the fuel, chances are it is already on "E", very rarely have I found a used car for sale with a full tank of fuel.
The buyers guide above is a very good reference.