I'm not sure what standard used 500's go for, but you can get a new one for around 250 after taxes and they come with three chokes. I beleive the combo models that come with a rifled barrel and standard barrel are around 3-4 hundred dollars.
If it is in good shape I think it's worth it considering it comes with two barrels and a hard case. If it's beat to heck you might want to consider forking up the extra cash and getting a new one.
Is the rifled barrel a cantilver for scope mounting or rifle sights and which would you want when deer hunting? If you want to mount a scope and it has iron sights I would pass. Even though the mossbergs come drilled and tapped for a scope rail on the reciever you would be much better off getting a cantilever barrel and mounting the scope to the barrel. More accurate and cosistant that way.
You could check gun broker.com to see what a 500 goes for used. If it were me and it had a cantilever barrel with it I would probably get it. If I had the spare cash that is.
Paul