Don Jose: There are basically two kinds of "kits".
The first kind are the ones made by Lyman, Thompson Center, Traditions and several other large companies.
These are basically factory gun parts which were pulled off of the production line at various stages of compleation.
The lock, barrel, trigger and some of the brass furniture are finished. The stock is inlet for the barrel, lock, butt plate etc.
The work consists of finish sanding the wood, finishing the wood and installing the hardware.
Sometimes little things like the fit of the lock, trigger etc needs some attention.
These guns are relativly easy to complete and are usually priced in the $250-380 price range.
The second kind (which is what your describing) consists of the parts in a rough state of finish.
Usually the lock and the set trigger (if used) are complete and fully functional.
The barrel usually needs to have the breech plug installed, the sight dovetails cut, the outside surfaces draw filed, the Flintlock vent or Precussion drum hole drilled, tapped and the vent liner or precussion drum installed, the underlugs dovetails or staple holes drilled and the underlugs installed, the outside surfaces finished and the sights installed.
The stock is usually rough inlet for the barrel, and lock but the final inletting of the barrel, the lock, the slots for the underlugs, the barrel tang will have to be done.
The outside of the stock will require anywhere from a little wood removed, to large amounts of wood removed.
The trigger will have to be installed, the length of pull determined and the butt plate cut made and the buttplate installed.
The thimbels, side plate, cross pin holes (for the barrel underlugs) the nose cap, patchbox will have to be inlet and installed.
The castings for the butt plate and trigger guard are rough sand castings and will require finishing and polishing.
There are a number of other things which will need to be done and I could go on, but by now you should have an idea of the size of the work envolved.
You should plan on spending at least 200 hours on finishing your gun.
If you feel up to it, go for it! It's a great hobby.