What are your likely ranges? if 50 yards or less, normal slugs are more than enough. longer? sabots and a rifled barrel, or maybe look into weather a muzzle loader is an option for you? can you do a magnum+ handgun?
I am not a huge fan of the stopping power of a 12 gauge slug. They will kill, but not nearly as quickly or as surely as a real rifle will. A slug will punch through fine enough, but a high speed rifle bullet will send a shock wave through the games systems and turn bone into shrapnel while they travel their path.
Buckshot, you aim for the head and hope a pellet hits the brain. basically like hunting deer with a a 22lr, but with more chances and a less precise aim. my least favorite option.