You can use .45 ACP as long as it's got a lead bullet and goes under 1000 fps. Ultramax reloads are perfect for this thing. Ruger BH Convertibles in .45 Colt often have the .45 ACP cylinders as well.
Here's the nuclear option for going as a CAS shooter:
2 Nagant Revolvers shooting .32 S&W Long. They have to be SA pistols, so all you have to do is disable that DA fly and you are good to go. Total pistol cost: 200 bucks.
Or 2 Ruger BHs in .357 Magnum. This assists with the kinds of ammo needed for the rifle. 800 bucks for the pair.
1 Shotgun, IAC makes a Winchester 97 reproduction that is SASS approved and runs about 300.00. As well as picking up a used SXS for 250.00.
1 Lever action rifle. I'd suggest something in .38 SPL or .357 Magnum as those calibers are cheap to find. rifle should run between 500-700.
Expect to spend anywhere from 900-1500 for a starter set of all the guns. Toss in some decent leather for 2-300, and regular clothing that can be made to look "Old West" and you are looking at 1200 (for ultra cheap) to 2000to start up as a CAS Shooter.
The good news is if you reload, the costs drop dramatically as the original cost in equipment are met once.
I have the 2 Nagant Revolvers: 1 a SA Soldiers model (150), and one converted DA (89.00), a Stoeger 12 Gauge Shotgun (250), and a Marlin .357 Lever action I stole (250).
Add into that the 800 I spent on the Taurus Gauchos (don't get them, they are junk) and we are talking serious cash.
It isn't for folks on the cheap. That's for sure.