To clarify for the OP, I think folks are talking two kinds of "kits" here.
There are "blackpowder starter kits" which is the gun sold with "everything you need except the powder". They usually come in a blister package with the gun, a tin of caps, some bullets, capping tool, powder flask, etc. etc. etc. All you need to do is buy some powder and you can go shooting.
Then there are gun kits where you buy the gun in an unfinished state. You have to finish the stock and metal parts and fit them together and put the gun together yourself.