.54 cal is sound advice. It's not a matter of "bigger is better", but .54 cal seems to be just the right diameter to retain more energy at longer distance than a smaller ball, while not having a larger non-aerodynamic surface to induce drag as it zips thru the air.
.50 cal is fine for most applications & you're not gonna be undergunned for most applications. I have several .50 cal rifles & they're all fine shooters. But comparing my .54 cal rifles, the .54 cal is easier to develop an accurate load with & is more forgiving for better throughout a wider powder weight range. With .54 cal, there's not much in North America that you can't drop with proper shot placement. .54 is comfy to shoot all day with moderate loads that produce great accuracy. Depending on which rifle, I typically load 75 - 80 grains of 2F black powder for 175 grain roundballs. I'm an 'old guy' and can shoot these loads all day. They will knock down any edible herbivore out to 125 yards. Beyond that, it's just too far to walk & anyway, you need to sharpen your stalking skills. My 410 grain hollow-base minie ball pushed with 110 grains 2F is no joy to shoot (a guy at my range shooting a .458 Win Mag told me so). Nevertheless, if you are hunting or hanging out in the same neighborhood with carnivores intent on eating YOU, it's plenty powerful.
The Lee mold is also sound advice. They're cheap, heat up quicker & produce quality roundballs, and they last forever. Detractors point at aluminum as easily damaged. My first Lee Mold was a .54 cal Improved Minie I began using in 1977. I still have it today, and it still works as new. A little cleaning and occasional lube & they're reliable for years to come.
When you get your rifle, first thing to do is crosspin the end of your ramrod. The glue, spit, threaded wood or sissy stuff they use to hold it together won't ...not permanently. There is no greater embarrassment (OK it's in the top 5) than gruntin' & strainin' to clean fouling outta your bore & have the rod separate from the tip. Drill a 1/16' hole crosswise thru the brass tip, then tap in a corresponding diameter finishing nail. If it's too small diameter for tight fit, just peen the ends, file flush & finish with fine emery cloth.
Next, disassemble your new rifle and completely clean it, especially the bore that comes coated with some noxious Italian bore preservative, which, if not properly removed before firing, bakes onto the bore surface & is a major PITA to remove. Carburetor cleaner is the best. Brake cleaner evaporates too fast.
Birchwood Casey runs counter to the advertisers here, but their Barricade is the best bore rust preventative made for a muzzleloader. It leaves a dry film & you don't need to swab it out prior to loading out in the field.
Everything else is sound advice for all the accumulated junk you need for your bag. If you shoot a lot, consider an Ampco / Treso nipple. They're tougher than steel & the flash hole won't erode to enlarge as quickly as steel. No hurry, though, since your factory Italian nipple should give you several hundred heavy shots fired before you even consider having to measure the flash hole.
The only important advice nobody mentioned was this: Seriously consider getting a flintlock instead of caplock. Flints are cheap, you won't need a nipple wrench & after a few shots are fired you'll overcome the pan flash flinch. Accuracy is extreme. My .54 cal 24" barrel rifle is flint & I routinely annoy other rifle shooters at the range by blowing up grapefruit at the 100 yard berm. All the pretty girls who come to the range to be ignored by Daddy or BF all want to fire a flintlock (when asked) and one even told me she knows what she wanted for her birthday (well, not THAT - she wasn't 18 yet).
It's just a suggestion, since once you master a flintlock, you can shoot anything else well.
