Right on! Glad to hear you're interested in the traditional route. You don't need a whole lot, but you do need the basics. If you're serious about round balls, I'd go 54 over 50 in case you want to chase elk too. Round balls are great killers, if you do some research on the traditional sites you'll find most guys get pass throughs on deer, full bore conicals are not necessary for deer. They are heavy and slow compared to a round ball.
Anyway, a good place to start is balls that are 0.010" under bore size, 0.490 in a 50 and 0.530 in a 54. You can go up to 0.495 or 0.535" if you have to, but most folks prefer the smaller ball and thicker patch.
Patches I'd start with 0.015", lubed 100% cotton. Or get dry patches and lube them yourself with bore butter or Hoppes BP liquid lube. I use the liquid for range time and greased patches for hunting. Pick up your shot patches and inspect them for tearing and burn through. You want a snug fit to engage the rifling and wipe fouling out. You might need to go up or down on patch thickness to dial in an accurate load with manageable fouling. Expect to need a stout patch and lots of lube to shoot hunting loads at the range without wiping between shots.
#11 caps are fine. You don't need a capper, sidelock nipples are easy to access.
Plinking loads I'd start with 50-60 grains, consult your owners manual for max load. Hunting load of around 90-100 grains is probably close, work up and down from say 80-max and find the best groups. These are for ff black powder, if you're using Triple7 or pyrodex read the label so you don't overcharge.
A short starter is helpful to start the patched ball down the bore. Your factory ramrod is likely fine, just grip it low and use short strokes to seat the ball. Or use a range rod, your choice. I recently started using a 1/2" dowel for a loading rod at the range. Cheap and sturdy.
The old Sportsman's Warehouse now Wholesale Sports in Lacey has a decent selection of traditional accessories. Otherwise, online sources are the way to go if you can wait on shipping.
For cleaning I'd get a nipple wrench, a bronze brush, jag, patches, and a 30-35cal brush if you have a patent breech.