All the Mossberg's on that site are in store only, but I'll search for an 18" one no homo
Just check out whichever regional sporting good store is local to you. Shouldn't cost more than $200 at the most. Maybe stick a light on it, but anything else isn't really necessary outside of maybe a pistol grip and a spare round holder. I'll post mine when i get home.
When it comes to guns, Lots of people, like myself, subscribe to the KISS concept of Keeping It Stupid Simple. Too many crutches will get you fukked up if something fails at the wrong time.