On the question of what choke to use with buckshot: In my 870, Mod or Imp. was fine. Full didn't seem to improve patterns like I'd expected.
I hunted a club were we ran dogs for several years. Shot a lot of deer with buckshot. I found that the plated/buffered shot patterned best; I preferred Win Supreme in 00 and 000.
Get some flipchart paper, pace off 20 yards, shoot. Do same at 25, 30, 35. You'll see the diff. On my shotgun, pattern starts spreading pretty thin after thirty yards or so, if memory serves.
At 25 yards, buckshot is deadly. Pretty good in the 30 yard range, broadside shot is best. Past that - over 40ish? - effectiveness plummets. Pattern you shotgun, remember it's a short-range proposition.
Good hunting!