I routinely get 3" groups with my Ithaca 37 12ga smoothbore, with the rifled barrel you should be able to find a load that it likes and get a good 1"-2" group at 100 yards. While the rifled barrel will shrink your groups an inch or more at 100 yards it will really shine past 100 yards.
Pretty much any reputable scope maker (Nikon, Leupold, etc, etc.) with a low power setting and long eye relief should do well for you.
Good luck.