as for what to buy, you have it already figured out, 870 or 500. some things to mull over...870 express is a fine hunting shotgun, get home from the field, pop the 26 inch barrel off, slap your mag extension and 18 inch barrel on, bang, now its an HD shotgun. 500 is also a fine gun, and the barrels are just as easy to swap out for being mission adaptable but the magazine tube isn't as it is plugged on the end, so if that is of conern, 870 is what you want.
As for plugging the mag for hunting, that only applies to waterfowl and migratory birds (as far as my state goes) so if you are hunting upland only, then that may not be a bother to you as well.
Other things to note; The ejector on a 870 is riveted in place and not difficult to break, its tough to do, but not impossible. The ejector on a 500 is held in with a screw and are very easy to replace. Not too much of an issue but worth mentioning. Accesories for both are plentiful and at most sporting good locations. 870 recievers are steel and 500 recievers are aluminum. I notice the difference in weight with the ones I own, your experience may differ.
Now, where to buy. Around here, pawnshops are the way to go. Seemingly eldless selections at very good prices. Most only need a little cleaing and 870's/500's can be had for $200ish in multiple locations. I just bought a mostly pristine 590 with speedfeed stock two weeks ago for $200 plus tax and got a box of 7.5 shot tossed in for testing from a local pawn shop.
Good luck!