There is a 12 gauge Model 12 at my LGS for $599. It is in very nice shape, probably 95% with a 30" barrel and 3" chambers. Is this a good deal or should I keep on looking?

Sounds like what was called "Heavy Duck"They were 3" chamber shotguns designed for shooting 3" magnum shells.You might do a search on Gunbroker to get an idea what they are going for,but I know they fetch a higher price than the standard 2 3/4" guns.

The correct pad is also important. You didn't mention a rib, so I'm guessing it has a plain barrel. That price, for a gun in the condition you described, wouldn't be unreasonable....if everything is original.

Heavy gun with solid Winchester pad lead weight in the stock and full choke with long taper. It was designed to shoot 3" Super X shot shells as indicated on the barrel. It should have wooden plug with "Winchester" in red paint in the magazine tube. The price is fair if the gun is all original. Great for Wildfowling if you don't mind paying extra for soft non-tox shot. You could use steel but the choke would have to be opened up to Modified or less.