First Tim, welcome to Greybeard.
Most everyone on this thread has a ton of muzzle loader experience and as you can already see, there is more than one way to skin the apple.
If you want to just see what it's all about, I could see a mountain rifle workin for a while. You have to remember though, that the less expensive rifles will not always work as well as the other rifles discussed. Sometimes, people get a bad experience with a rifle that wasn't made as well as another similar brand. It turns them off black powder shooting and we loose a convert. So we try to ensure that a person's first experience is a good one.
Be sure to carefully check the barrel of any used rifle that you buy. Run a dry patch down the bore with a cleaning jag. Feel through your fingers on the ramrod how rough the bore is. If there is any roughness, it indicates possible pitting of the bore and a bore light is necessary.
Whatever you get, be sure to find a good book on the subject and go to a range where blackpowder is shot. Find a person shooting a sidelock, walk up and ask for a little guidence. Most true blackpowder shooters love talking about it as much (almost) as shooting it.
I have had over 30 years of fun with muzzleloading. I wish upon you the same.
C F