shotguns are wonderful tools; very versatile and dependable in many uses and situations; I would hold off putting together a rifled barrel & scope setup for your thumper until you have a few seasons under your belt; you are making a good choice sticking with slugs meant for smoothbore barrels; Winchester, Remington, and Federal make good slugs that has softer lead than Brenneke; this promotes good reliable expansion, but deposits lead like crazy in your barrel; Brennekes are harder due to a higher antimony content and this makes them excellent penetrators and this also helps with lead depositing in your barrel; if you want to advance to saboted slugs and don't want to spend the money on a new barrel, you can get a rifled choke that screws into the muzzle of the barrel; the rifled choke isn't quite as accurate as a fully rifled barrel, but the choke emparts a decent spin that stabilizes the slug better than no spin at all;
I got in over my head and assembled a fully rifled barrel and scope setup, but had problems with the quality of the other barrels used with the shotgun...I traded the shotgun and the problematic barrels and sold the rifled barrel and scope separately...I had a whole stash of Lightfield rifled slugs leftover, so I opted for a rifled choke tube for the used Browning BPS that I got to replace the plagued Remington 870; I can hold a 2 1/2" to 3" group unsupported offhand at 50 yards with a hiviz bead on the barrel rib...not the best, but enough to whack a deer or bear during hunting season; my setup is a backup incase anyone in the hunting party has a weapons malfunction that can't be rectified;
as far as HD buckshot choice...#4 buck is about the minimum for serious use; I live in an apartment and keep a stash of Sellier & Bellot #4 buckshot in the house; my father (who is coming to live with me) and I opt for multiple loaded revolvers in the domicile since the space inside the apartment is cramped with a full length field barrel on the shottie; many feel that birdshot will produce ugle and gruesome wounds, but will underpenetrate and may not nullify the threat; your choice of ammo will depend upon domicle construction, occupants, distance to the next building or proximity of neighbors in the next apartment, ability to place followup shots under repeated firing, etc, etc (if you opt for buckshot, there are reduced recoil loads available)
best of luck and enjoy!