Muzzy kits can be anything from completed guns that just need sanding and finishing to a box of parts and a stock blank.
I think most of the ones people are talking about here fall into the first category and they're probably the best choice for a first attempt. If you're a little bit handy you get a nicely finished piece with a bit of a personal touch. They're easy enough to do (if I can do it anybody can) just take your time and don't rush things. No special tools required beyond screw drivers, files and sandpaper.
There are also middle ground kits, most of the inletting is done, but things still need to be final fitted, screw holes drilled and tapped, tennons cut,etc.. They tend to be more historically accurrate and better quality but you're looking at more assembly time and you should have a pretty good idea on how a muzzleloader goes together as well as the tools to do it. FWIW, I tried one of these and while the rifle was functional, it pretty well convinced me never to do it again.
The box of parts and blank piece of wood... forget it.
Here's a link to a gun building tutorial by a master builder. Well beyond anything most of us would attempt but you can see how they're put together.