As far as the "which rifle" question goes, if you get twenty responses to this post, you'll get twenty-five different answers. Firstly, What kind of shooting are you going to do, SASS, NCOWS, 3 Gun? It makes a difference. The general consensus for SASS is if you want the fastest gun in the west, get a Winchester 1873 and have it short stroked and slicked. Be prepared for a hefty price tag though. If you are on a budget, get a Rossi 1892. Durability? Marlin. Everything else is somewhere in between.
The 1897 is a great shotgun, but they break fairly easily and CAS is very hard on them. One advantage though is that you can use it for Wild Bunch shooting too. If this is a family heirloom, I would get somethng else. Like I said CAS is HARD on the 1897.
The calibers are your choice. The fastest guns in the tournament will be shooting smaller caliber, less recoil, less muzzle flip calibers, .38 Spec. is popular. The traditionalists that will scoff at you (with a wink and a smile) will be shooting 45 L.C and 44/40 and loving it.
Don't worry too much about the costume. The only time that it really matters is when you want to shoot "Traditional Cowboy" or "B Western" The rest of the catagories are pretty loose,especially for new people. Of course the Wild Bunch folks can get a little bit picky sometimes too.