A ProStaff scope will be fine. I had one on my old Remington Genesis and liked it. But the coolest thing IMO about a muzzleloader scope is the added eye relief. The Nikon Omega I have on my Encore Endeavor had a lot of eye relief. 5" I think. My face is nowhere near the scope when I pull the trigger. This is great for heavy recoiling weapons like our muzzleloaders.

As far as accessories... What I have found is that you can't just go out and buy it all at one time. You will accumulate your favorite and most useful accessories over time. I would just get the basic cleaning stuff now and then as time goes by you will acquire more and more stuff.