I could write a book on this subject...I collect older shotguns, especially the Browning Superposed shotguns and you can find a great deal of older Browning's for reasonable money, but you can also spend a pile of money. I have several Brownings, many of which see range time regularly. I prefer the over/under configuration myself, but also have a 1934 Winchester 21 Tournament Grade that is pure joy to shoot. Also look at the entry level Connecticut Shotgun lines (American handmade beautiful guns). The newer Browning Citori's and Ruger Red Labels and Winchester 100 are also good choices!
What specifically do you want to do with it? Older shotguns can be great investments...