Just find a used, nut not abused plain jane model 500 in 12ga, say on gunbroker.com or one of the many other auction sites or from a local dealer and convert it. An 18" security barrel with cylinder choke and bead site, around $70 bucks. You can get an ATI collapsible stock for around $50-$60 bucks.
You also might consider the Mossberg JIC ( Just in Case) series. They go for around $275-$400. Not sure if they all or none have the breach piece on it. These two models don't.
http://www.gunbroker.com/Auction/ViewItem.asp?Item=88896057
GunBroker.com guns: Mossberg 500 JIC Just in Case Sandstorm 12 gauge (item: 89281716 ends: Jan-20-08 04
04 PM)
G-Meister