The best thing you can do is to attend a couple matches as a spectator. Talk to the Cowboys & Cowgirls there to find out what they are using, what catrgories they are shooting in. ASK QUESTIONS!!!!! You will find that everyone there will be friendly and answer all your questions. SOme will even insist that you try out a coupel stages using their guns.
CAS won't necessarily mess up the finish on any gun. It all depends on how you treat them. Use will cause wear.
With the Win 97s and Marlin 94 you have now; Picking up a pair of single actions in 44 Special or 44 Mag would be the logical step. Keeping your rifle and pistol ammo the same lessens the chance of foulups (don't ask me how I know).
A pair of used Vaqueros or Ubertis would be a good start for beginning six guns. You may decide to change guns/caliber/category after you've been shooting a while, so getting started as cheaply as possible will help.
Fingers (Show Me MO smoke) McGee - AKA Man of Many Colts - Alter ego of Diabolical Ken; SASS Regulator 28564-L-TG; Rangemaster and stage writer extraordinaire; Frontiersman, Pistoleer, NRA Endowment Life, NMLRA, SAF, CCRKBA, STORM 327, SV115; Charter member, Central Ozarks Western Shooters
Cynic: A blackguard whose faulty vision see things as they are, not as they should be. Ambrose Bierce