If you stick with the smoothbore barrel I would not take a shot over 75 yards, improved choke will work fine and considering the number of slugs you will run through the barrel I doubt you will see any extra wear on the barrel, I have never heard of a smoothbore being hurt shooting rifled slugs, but that may not be true for all of them.

The Tazman aka Martin Price
Founder and President of
Virginia Disabled Outdoorsmen Club