The Mossberg will serve you well and since you already have it, I'd use it unless you just want a muzzleloader. As with any firearm shooting a single projectile, you'll have to experiment with which ammo shoots best from it. If the barrel is rifled, you don't need rifled slugs. Try Sabots from different manufacturors.

The Omega is one nice gun. But you'll still have to find the bullet it likes, and, you'll have to find the right powder and weight of the charge that it likes as well.

I'd get either the Nikon or Leupold scope that you like best. Make sure it's made for shotgun use (ie long eye relief!!!!!) and will withstand the recoil. I'd opt for a 2-4 power scope.