80 grains is a good start, and yes the rifle will shoot powerbelts. I personally shoot a 300 grains XTP out of my staghorn magnum which is what the buckhorn replaced. Same rifle as the stagborn with an extra locking lug and a nice stock.

Mine shoots best with 90 grains of Pyrodex RS or black powder, but Triple Seven should work well. Swab between shots and clean it good at the end of the day and that rifle will serve you well for many years.