The Stock Inlet and Drilling refered to is the inletting of the barrel, the lock, perhaps the trigger.
This inletting is what I would call "rough inletting". The final inletting will have to be done by you.
What this means is you will have to remove some wood everywhere to get the parts to fit.
Actually, this is good because it allows you to obtain line to line custom fits between the wood and the metal parts.
These "kits" take a lot of time to do correctly.
I believe the drilling Rich was talking about includes locating drilling (and tapping) the holes for the screws which hold the parts on. These would include the lock bolt and the tang bolt.
I haven't built this gun, which is called a "Bar Lock" but I do know the location and fit of the lock and barrel have to be exact to look and function like they should.
I have several thoughts about your selection for a first time build.
These "kits" are just a bunch of semi finished parts. They do not just "fall together" like the kits you could get from Thompson, Lyman, Traditions etc.
Thompson, Lyman, Traditions kits are great for first guns because they teach some things and about 3/4 finished before you start.
As Rich says, it can be done. It is also a major project that can't be hurried. If you are willing to take your time, buy and read some good books on building guns, ask a Lot of questions here on this Forum and buy some hand tools you may not have now, you could do a nice job.
If I were to recommend a few styles of these "custom" guns in addition to those already mentioned for first time guns, it would be a plain Southern or Tennessee rifle in Flintlock.
Flintlocks are not as fussy about things like lock to barrel location so they are easier to build.
You could also order some of the guns from other dealers which are basically "in the white". These are not too much harder than the Commercial companys Kits although the selection of the style is somewhat limited..
Let us know what you decide.