Barrel length is the least of your concern, and doesn't really matter at all. To get good accuracy at 80 yards from a rifled slug requires either a good slug gun, or rifling. A rifled choke tube would do it. You aren't going to do it with a basic smoothbore barrel that you slap on your frame. A vent rib barrel with clamp on sights, the gold standard is 3" groups at 50 yards. Most such guns will only shoot 8"-12" at 100 yards. Rifling ruins buckshot patterns. If you accept you will only have an effective range of 50-60 yards, then your criteria is ok.
Buckshot can be hard to predict what works good. A moderate or low velocity load generally patterns tighter. Buffered loads almost always pattern tighter than non-buffered. Federal flightcontrol buckshot is a great option, but there are others out there. There are special buckshot choke tubes, but I'm not sure how many are rated for slugs. I would stick with standard modified and full chokes. There is no standardized pattern test for buckshot. For sure you will want to pattern a minimum of 3 shells, 5 being better. Buckshot can vary, although the best quality loads are pretty consistent. For deer, size can depend on your laws. In Wyoming 00 is the smallest allowed. 4 buck is used in the southern states, but Ive only ever used that on coyotes. 1 buck and 00 buck are the most common you will find in 12 gauge, and both will be good. 000 might be an option too. Shot size can be a big factor in how a load patterns. I personally would want to average a minimum of 3 pellets in a 10" circle. This will probably be 35-40 yards for you, but can be farther.
