KISS
12ga pump under $300. Winchester, Remington, Mossberg are the primary makers. 18 1/2" Barrel and buckshot. Check the local gunshops and pawn shops for police trade ins, these are 'combat ready' usually and generally under $200. Hand pick the best one if you go that route.
Keep the full stock, or get a pistol grip with full stock, but do not get just a pistol grip shottie. Consider a gun mounted light for it.
