Buy yourself the rifled/cantilever barrel or like you said the whole gun with the rifled/cantilever barrel, you will get much better performance versus the smoothbore barrel. Mossberg is a proven shotgun and a proven company.
Look into a decent scope, for the money the BUSHNELL TROPHLY scope in 3-9x40 is very good for this set-up.
Shotguns are very iffy with slugs, 2 of the same model/brand may like different slugs, try different brands of saboted slugs, but seems like the LIGHTFIELDS work great most of the time