While low mileage is usually good, if the mileage is extra low it might mean that the car was stored for an extended length of time. Which can be just as, if not more damaging than having higher miles. Look for something ranging between 6k-12k miles per year the vehicle has been on the road.
The next thing to look at, when you find a car you like, is the colors of the fluids. Generally the fluids should be transparent to semitransparent for the most part, with no sediment in it. You can go to a local auto parts store and check out the fluids themselves to see what new fluid looks like. Very dirty coolant, and oil that looks like chocolate milk are huge warning signs to walk away!!
For the body: Dings, scratches et cetera are not a problem unless they are into bear metal and rusting. Thin surface rust in an area less than a dime in size can have touch up paint applied and will probably be of no consequence. Large areas of rust or bubbling paint are no good. Pay special attention to the areas around the wheel wells and the bottom of doors.
To find a mechanic: there is a very nice feature on this forum's parent site; the mechanic files. Check them out here: http://www.cartalk.com/content/mechx/
Many times state certified means that the car was in a wreck and has spent some time on a frame machine. If you are buying a car from a shop that has repaired it expect this. You can get great deals this way, look to pay 60-70% of blue book value, no more. Make sure that the place in question has a good reputation. I have owned 2 vehicles purchased this way and was very satisfied with both.
As for finding a car, I still think that one of the best places to look is in the local paper's classifieds.