Hi Praridog, there are many members here more knowlegable about this type of shooting than I am. But I can give you my ideas that have developed from reading many of these posts and shooting my rifles.
First, are you going to shoot round ball or conical bullets.
Minie balls, maxi balls are all heavy long projectiles that carry great energy down range. BP guns with these bullets are very close to the big lever action rounds. 45-70 50-70 etc. Recoil is a real factor in the larger calibers 54 and 58.
Twist required is usually 1x48 or quicker. 45 with the conicals give the best energy for the recoil generated in my opinion. In conical bullet shooting you can increase the length vs bore ratio and get tremendous increases of kenetic energy and penetration power.
Recreat the balistics of the 45-70 and you will not be disapointed.
Roundballs are the cats meow in my opinion, they are accurate and not as bad on the recoil. Twist generally needed is 1x48 and slower. Specialised round ball barrels use 1x56 to 1x72 and even slower if you want massive powder loads.
I love the .58 caliber roundball, it seems to just over power the lesser calibers and not beat you to death like the .60's .62's .69's etc.
Larger Bore diameter and larger powder charges are the only way to more power, wind resistance and retained energy at longer range.
Patch, lube, wad, ball configurations can all be adjusted and lead to many enjoyable days at the range. Many are discussed in the historical posts.
Roundball, a member here, turned me on to using Oxyoke wonder wads over the powder then the lubed pillow ticking patches from Oxyoke. The wonder wads keep from burning your patch with hot hunting loads, target loads usually dont need them, but let your rifle tell you what it likes.
Real black powder is the only way to go, no substitue for the real deal. But you can get good results with the subs and a caplock rifle, flinters require BP to function correctly. There are several good powders to choose from, GOEX, Swiss, Shutzen, KIK etc. Several firms will deliver to your door at very reasonable prices. Fed Law limits individuals to 50 pounds or less. Best price is in 25 to 50 lb case lots.
This brings up another lesson. Just as in smokeless shooting lot to lot variance of powder causes problems. Buying case lots helps with consistancy.
I like the T/C Hawken / Renegade series of rifles which you can buy after marhet barrels for in a great variety of twists and calibers. They are very versatile.
Hope this info helps and read the historical posts, they are a mine of information. Then go shoot and shoot some more. Have fun, I know I am.
Hank