Personally, I chose a .50cal. and I'm not sorry.
When looking at used rifles, go over the wood extremely close. Look for cracks and repairs. If it's your first one, walk away on anything that looks like it broke and was repaired. Later on, these are good for project guns and can be had for a good deal if you work it to your advantage but for the first gun, don't mess with it.
Bring a strong penlight with you to look down the barrel. If you see more than bright shinie metal and a clean rifled barrel, turn it down. No pits, or debris for you. Black powder is corrosive and if the owner didn't put the gun away clean and is showing it to sell with a dirty or messed up barrel, it's not for you.
Is this a flint lock or a cap lock rifle?
Either way, have him show you it works by setting off a cap or two or sparking the flint.
Accessories, if he has goodies, ask for them!!! Work the deal to include them or get them at a reduced add on price.
extra balls/patches and POWDER!