I highly recommend the Uberti-made copy of the Remington New Model Army. This is also mis-named the "1858". The Remington design is sturdy, accurate, and reliable, and very easy to take apart and clean. Uberti makes a "millennium" finish (often called "matte" in catalogs) that resembles black parkerization. It is reasonably tough and rust-resistant.
As a general rule, skip the brass-framed guns. They are not very sturdy and are more wall-hanger than shooter. But if you are only going to shoot this gun once or twice a year, 10-20 shots per use, then a brass frame will last long enough.
BP guns must be cleaned after firing, the sooner the better. Leaving overnight is a bad idea. I try to clean mine at the range, or upon arrival home. Hot soapy water works best for cleaning. Ballistol is the best oil for these guns.
American Pioneer Powder (APP) is a good black powder substitute, that is also much less corrosive than real BP or Pyrodex. Never, ever, not even once, use any modern "smokeless powder" in a cap-and-ball load. The gun will explode.
Different guns work best with different percussion caps. For example, my Uberti Remingtons and my Ruger Old Armys work best with Remington #10 caps. CCI #11s will work, but are too loose and have to be pinched slightly out-of-round to stay on the cones. CCI #10s won't fit on the cones at all.
Load the recommended powder charge, typically 24 grains by volume for a Remington. Use a .454 ball in a ".44" (and a .457 in a Ruger ".45"). The ball should be shaved of a small ring of lead. If you have cast lead balls, the sprue (flat spot) faces up out of the chamber when loading. Hornandy brand balls are swaged, and have no sprue. Seat the ball all the way down on the powder. Firm compression for BP, light to almost no compression on APP or Triple Seven. Seal over the chambers with BP grease (Crisco in a pinch). This keeps real BP from forming hard fouling in the barrel, and helps prevent chain-firing of multiple chambers. You can substitute a pre-lubed felt was for the grease. Grease goes on top of the ball. The wad goes between the powder and ball.
Load five shots, not six, and rest the hammer on the empty chamber. The hammer would rest on the percussion cap of a loaded chamber under the hammer. Striking a mild blow to the hammer would fire the gun. Load the chambers, then cap the chambers as the last step.