Look around at
https://www.sassnet.com
on the SASS Wire forum and the affiliated merchants links.
I don't know about the differences between the grades of AWAs but there is plenty on the SASS Wire, pro and con. The speed shooters tend to the shorter barrels; probably not much difference between 4 3/4 and 5 1/2 inch, but my 7 1/2" guns are a little slower. But I shoot Duelist, and am not real quick anyhow, so a Clean Sweep is my goal, not a teriffic time.
Marlin lever actions have an advantage for black powder; just remove the lever screw and the lever and bolt come right out. That is very convenient for cleaning. My BP shooting pard can clean his guns faster than I can get the nitro fouling, hard wax lube and leading out of mine (BP fouling and the soft lubes for it wipe out with Windex and proper BP loads do not lead, period.)
The .44-40 is said to have an advantage with black, but there are plenty of .45 carbines being shot with black, including my neighbor's. I shoot .44-40 only because I got a great deal on a Winchester '92. It is a bit tedious to load for and if I were using all new guns they would be .38s or .45s.
There are many, many types of holsters. Get a look through somebody's Cowboy Chronicle and see a bunch in match pictures and advertisements.
You also need a shotgun, double barrel is going to be easier to clean up after black than a pump. Probably a cart or even a kid's little red wagon to hall four guns, leather, and ammo in.
Look at all the pictures for costuming ideas. You don't have to start out looking like an extra in Tombstone, plain jeans, work shirt, hat and boots will do to start. From there the sky is the limit. Costume trophies are as tall as shooting trophies, and just as prized.