I used to build ML's from scratch, meaning buying a barrel blank, lock kit, stock blank, trigger guards and butt plates, making my out ram rod ferrules, nose caps, side plates, patch boxes, sights, etc.
Some kits are very good, some are not. I would suggest you contact some folks in your area who are members of a muzzleloading rifle club and get their opinions on the kits currently offered.
I once had to recut the rifling on a CVA squirrel rifle, due to there being a slag inclusion in the bore. I wouldn't recommend that for the first time builder. LOL
