The great thing about starting shooting a black powder, and a handgun in particular, is the relative cheapness by comparison with any cartridge-firing handgun, which, for most of us, is not a likelihood unless we live in Norn Iron.
True, you CAN buy everything new, but there are oodles of good used guns out there, often with all the accessories you might need to get you started, apart from a BP license, that is. If you decide to go the substitute powder way, then you don't need either the BP licence, nor the specialised storage box for it either.
You could start by asking around the club, and then your local dealer, about anything that might be available. Most folks seem to be drawn to a .44cal revolver, but its true to say that there has never been a more ergonomic revolver on earth than the Colt .36cal Navy. Powder charges of around 18gr - just a bit more than half that of the .44cal - and a ball around half the weight, mean that you'll get LOADS more shooting per pound of powder and lead with the smaller calibre.
Ask away - there is a ton of experience on this forum, but the first thing that you MUST buy is the Lyman BP handbook - the best money you'll ever spend on a gun-related book if you have any interest whatsoever in BP shooting an bullet casting.
tac - BP shooter since that was all there was.......