At most matches you will need two revlolvers to compete. That said,it is common for newcomers to the sport to borrow any guns they don't have from other shooters. CAS requires a pretty hefty outlay to come up with the required arsenal. At any shoot you go to,you'll find folks willing to loan you what you need. The best bet is to contact the match coordinator of the shoot you wish to attend and mention that you're missing this gun or that gun,and they will set you up.
As far as C&B guns go,it's really personal preference. I have an 1858 Remington. Some folks prefer these since they have a backstrap and are inherently a little stronger than Colt designs. Colts come in more varieties,so finding one of those that fits your hand may be the determining factor. Since the cylinders on Remingtons are very easy to swap out,getting two or three cylinders loaded can save you some work throughout the match.