I got bit hard by the CAS bug about 15 years ago and seldom shoot any other discipline anymore.
The 1873 is probably the most used rifle, with the 1866's and Marlins next. A Rossi 1892 is likely to be the most affordable. Like a lot of folks, I started with a '92 in .45 Colt, then went to toggle link '73's and '66's for a few years, then started shooting B Western a lot and have been using Marlins almost exclusively the past few years.
Pass on the Winchester 1894 - it was made for a longer cartridge and usually doesn't work well with revolver cartridges. I've seen a few Henry Big Boys at the matches, but most folks would suggest passing on it too. Likewise, also pass on the Lightning pattern pump guns, ESPECIALLY the Taurus.
Ruger Blackhawks and Vaqueros are a great choice for main match revolvers, but there's certainly nothing wrong with Uberti's either - I have 31 of 'em.
The cheapest revolvers would undoubtedly be either brass framed percussion guns or single action 1895 Nagants. Black powder revolvers are probably not the best choice to start out with, and I've NEVER seen anybody shoot a Nagant at a match.
Most any SxS shotgun w/o ejectors will work fine and are likely the most cost effective, other than a single shot. Single shots with or without ejectors are legal, and I use one every once in awhile just to be different.
Some categories call for different guns: Fixed sights or .40 cal or larger or specific types of shotgun actions, etc. For shotguns, a hammered double, single shot, or 1887 lever action is legal in all categories. Fixed sight revolvers of .36 cal and larger are legal in all categories. Fixed sight cartridge revolvers of .40 cal or larger are legal in all categories except Frontiersman, which calls for percussion revolvers. Classic Cowboy calls for a rifle from the time frame of 1873 or earlier, while B Western specifies 1880 or later.
The online shooter's manual can be found
HERE.
Noz, you have the '87 under your heading of Pump shotguns.
(It's a lever action shotgun, and I know he's aware of that.
)
Now... I have a match Saturday, and two to choose from Sunday...