In order to help we need some more information. Will you be hunting deer with slugs or buckshot? If using buckshot then get a 12GA. If you will be using slugs then a 20GA is more than enough. Also some states specifically state "Smooth Bore" while others allow use of "Rifled Barrel". Where are you located? Will you be using the gun just for deer or other game as well? For what you pay for a used gun you could go to WalMart® or Big5® & buy a new Mossberg®, Remington®, or Winchester® pump.
Give us some more information & all the guys & gals here will try to help you. Regards.....
Boy, David was SPOT ON with his answer. It could not have been put any better. We would like to help you, but the is just hardly any way without having the answers he has aked for. No offense, but your question would be about like asking, "I want a car. I will be driving it down 3rd St. What should I get?