All of the above and also check with your state's muzzleloader requirements. Some states have restrictions on type of ignition (flint, percussion, other), type of weapon, sights, bullet, etc. Check all the laws.
If you intend on buying used, it may be wise to invest <$10 for a bore light by CVA. This little light slides down the bore so you can look at the rifling and avoid buying one with arusted or pitted bore. (tip - put some masking tape over the light so it isn't too bright that it obscures any defects in the bore).
Also, you may have your own personal preference as to the type of ML you want. Do you want an inline or a sidelock?
Whatever you decide on - continue to ask questions here. Most all of us are willing to give you advice. (and most of it is pretty sound).