Lots of good advice given here already, I'd add 2 things.
1) On the Mossberg you're looking at, does the rifled barrel have a cantilevered set up so that the scope attached to the barrel rather than the receiver? If it doesn't, you have to attach to the receiver which is an awkward set up -- hard to switch back and forth between barrels, and maybe some accuracy issues.
2) Optional equipment, though good to have: Best $10 I ever spent was a "Butt-Out" tool from Hunter's Specialties. Makes the hardest part of gutting a deer very easy.