Another thing to watch out for is a table that has been badly maintained. Check all the pockets to ensure they are held on with only machine screws. On a GC if you see any nails holding them in it's a sign of sloppy table mechnics, and the nail heads will scratch that nice new set of Aramith balls you buy (You are buying a new set of balls right?). You'll probably want to budget to replace the pockets too on a used GC, as they wear the most of any part except the cloth.
As mentioned above the rail rubber is critical. Note that although it is relatively cheap to buy replacement rubber fitting it properly requires an expert table mechanic. Get the best quality rubber you can afford, ideally Artemis. Getting cheap stuff will always come back to haunt you in the end.