5-shot Mossbergs can be found in any pawnshop, flea market, or used-gun rack in any part of the US for cheap (None of my 5-shot Mossys cost more than about $125) whereas the 7-shot models are popular with the Tacticool crowd and they drive the price up over $200.
Buy a complete working but cosmetically challenged 28" smooth barrel 5-shot Mossberg for under $125, saw it down to 18.25". You're in the deal. Spend the money you saved on more shells and spend more time at the range practicing.
Heck, you'll pay close to that for just a barrel for the 7-shot.