I don't know what it is, but any time you can pick up a functional firearm for $100 that's a damn good deal.
Black powder percussion cap muzzle loaders are selling new today for $800 and up. The used market puts them around $500 and up. So I would not hesitate to spend $100 on it even if it was just a wall hanger.
If it is an ArmiSport, you might be able to get information about the rifle from them here: http://www.chiappafirearms.com/
One way to determine the twist rate is to put a patched cleaning rod in the end of the muzzle and allow it to spin as you push it down the barrel.
Make a mark on the side of the cleaning rod where you start, and then another mark at the same spot when it turns 360 degrees. Measure the distance between the two marks and you now know the twist rate.
There is a variety of ammunition available for the .54. You can shoot patched round balls, Solid projectiles that engrave the rifling at loading, or expanding balls (minie balls).
The first thing to do when you get it is to drop the cleaning rod down the barrel to make sure it isn't loaded. You should get a nice metallic ring when it hits the breech face. If you hear a "clunk" you might be hitting lead.
360 degrees was 180 degrees.