If you have any thoughts toward putting a scope on your shotgun, make sure you get a barrel with a cantilevered scope mount.

You'll be fine with a Mossberg 500 or Rem 870, though I've heard some complaints about 870chambers needing to be honed for consistent ejection.

I feel that 3" shell capacityis adequate, no need for 3.5.

Consider used guns for your first purchase. Mossberg 500s and Rem 870s last forever, and unless there are obvious damage issues, a used gun should serve you well enough to get started.