Forget the single shot. Like everyone else has said, a Mossberg 500 or a Rem 870 is the best thing going. Take your pick on gauge, but I'd go 12. Simply for the ammo selection, and cost. If you're on a budget, 12ga is the way to go.
Also, screw in chokes are invaluable, IMO.

If it's still there, I know where there is an older 870 Police Magnum for sale, and it has a brand new rifled slug barrel on it, for $250. Pick up a shot barrel at the gun show or pawn shop for $80, you're good to go.
OR, a guy at work today told me about a Mossberg 500 combo, for $250.

http://www.armslist.com/posts/626790...gauge-12ga-28-
http://www.armslist.com/posts/644249...870-wingmaster
http://www.armslist.com/posts/596268...g-shotgun-12ga
http://www.armslist.com/posts/596177...s-shotgun-12ga