I just went with the Benelli M2 Field 28" with CT stock. I felt that was the best option for a 12ga that is reliable with light loads for clay or upland game up to 3" magnums for hunting quackers
The A300 is a solid choice. One nice thing about the A300 is it has an adjustable LOP to help make the gun fit you and screw-in chokes. A used Beretta 391, Remington 1100 or 1187 are good choices. Personally I would find one with screw-in chokes it gives you flexibility that a fixed choke can't. Stay away from the Stoegers and other cheapie guns, you get what you pay for. No matter what gun you choose (assuming it is in good condition) fit is the most important thing.