You are very unlikely to find a pump that's worn out. Do your basic function test-trigger, safety, make sure everything cycles ok, ie the shell lifter pops up at the end of the stroker and the shell release moves to allow a round to pop out. On the "big four" as Dave calls them, parts are easy to find and cheap if you do happen to buy a problem child. Dave's "big four" IIRC, are:
Remington 870-the Wingmaster has nicer wood & better finishing than the Express, and so commands a premium
Mossberg 500-generally considered to be on par with the Remington, but cheaper and IMHO it shows a little
Ithaca 37-new ones are being made again, but they are expensive. This is a turn of the century design (one of JMB's) and pretty unique. They load & eject from the same port on bottom. This is great for lefties, but means a shell has to be put in the magazine before it can be chambered....no slapping a shell in the ejection port. Old models made before a certain S/N (is it 855,000?) don't have a disconnector, and can be "slam-fired" nearly as fast as an auto. These older guns however, have hand fitted barrels that can't be changed out.
Winchester 1300-no longer produced. I have one & love it, it has a more complex rotating bolt which gives it inertia-assisted ejection; it will eject the empty on its own if you fire it one handed. Parts such as barrels are much harder to find. The new Win. SXP appears to use the same bolt design, but with a different barrel. I haven't heard much about them yet.
The Benelli Nova & Browning BPS are also earning very good reputations, but will be more expensive.
NEF makes great single shots, but I haven't heard anything either way about their pumps. That said, I would buy a used of any of the above over a new NEF, Maverick 88, or Chinese clone any day. Shotguns are one of those places you almost always get what you pay for.
Also, don't overlook a good used auto if skeet or sporting clays will be in your future. Pumps are great for trap & HD, but are at a slight disadvantage for most games.