What you will find is a large discrepancy in the quality and value of a good used piece versus a new one within the price range you are looking. Granted, your intended use probably won't result in hundreds or thousands of rounds through the gun in any given year, but the higher quality guns will provide many, many years of troublefree service with better fit, finish, balance and ease of operating. The lower end guns (Tristar, Baikal/Spartan, etc) are solidly built pieces and are capable of lasting for many years, but simply are not made to the quality of something like a Browning, Beretta, Ruger, etc.
To check the "basic" condition of an O/U simple remove the barrels and inspect the bores for pitting or scratching, inspect the chambers for rust or rough places, hold them vertically from the chamber end and tap or thump the barrels- you want them to "ring" like a tuning fork then look for dings or dents along with the general condition of the finish. Then inspect the general condition of the receiver looking for rust or damage to the water table, hinge pin and trigger. A reputable gun shop should be able to verify the actual condition and servicibility of a used piece and provide you with honest advice concerning the purchase.
"Shoot low boys...they're riding shetland ponies in search of true grit" Lewis Grizzard