RE: buying a old shotgun
I don't think you could even get a rifled barrel for that gun. And if you did find one it would probably be used and cost more than you paid for the gun.
I could be wrong though, it's been known to happen.
If you are even remotely considering getting a rifled barrel stick with the 500 or 870 if you are on a budget. Nothing wrong with used though if you can get a good deal. The problem is in most cases for not much more you could get a new one if you shopped around.
Paul