want to second the friendliness of CAS shooters, personally i only have experience with SASS (single action Shooting society) matches, they are the most prolific and well "forgiving" with regad to historical accuracy of guns and clothing used/worn. there ARE other groups that are more historically correct in equipment but SASS shoots are best to start off with from all i can gather.
at the first regular match i attended, i showed up simply to watch and happened to have on my boots along with my regular clothes (jeans and a long sleeve cotton shirt). but had only brought along the rifle that i had intentions of using for my match gun (i brought it so i could stop by my "home" range between the match adn my house and run a few more boxes through it ). about half way through the runnign of the first stage by the "posse" i'd chosen to watch i was approached by the club vice persident and asked "why ain't you shooting??!!" i told him "ain't got all my guns, nor any gunleather or other gear", to which he responded "that ain't no excuse, we can get you gear and guns to use! your gonna shoot today. on;ly question is am i gonna have to whoop ya to get you to get over here?!!
needless to say i had a blast, adn if not for losing my job back in Oct.
and having to sell what gear i had accquired, to make ends meet, i'd be making plans to shoot my first match with all my own stuff now.
that being said my oppinion on starting gear (if you don't already have favorites within the gun makers) is this, first thought adn this applies to each of the three gun types, new or used don't matter much they're YOUR guns after teh money passes hands. Ruger pistols (blackhawks for modern, vaquero for all others), a Marlin 1894 Cowboy, great gun for teh price and it just feels more "right" don't bother with the "Cowboy competition" model till you're sure of what you want, on shot guns you have two choices that are near the same price adn contrary to what many will tell you there isn't much of a compeditive difference eaither.. you can go the SxS route and get a Stoeger Coach gun, or the pump gun route and get a Norinco copy of the Winchester '97 coach gun.
as for leather good i recomend the good folks at
san pedro saddlery Big Ed (the owner) will treat you right adn make sure YOU are satisfied with his goods, and there is a "new shooter package" discount deal where you buy 5 pieces and get free shipping and 10% off. the 5 pieces most folks get are .. Gun belt, two holsters (unless you shoot Gunfighter class, get a 1 strongside adn one crossdraw) , a "speed loop" shotshell belt slide, and a Badge holder (fr your SASS, badge)
for more info i would suggest visting the SASS website
SASS and go to the SASSwire forum adn ask any questions you have over there as well.
sorry for being so long winded, hope i am of some help