Your choices sound good. A couple of comments on them.
I don't know what your machine/mechanic skills are so I'll just make a comment. A lock "kit" usually means that you will be drilling and tapping holes for the bridle and sear spring, etc. A "finished" lock of the same style will usually run around $25 more. You have to gauge how much time you want to put in to assemblying the lock from a kit as well as your skill levels versus forking over an extra $25 for a finished lock. In the past, I've assembled locks from "kits" but the lockplate was already drilled and tapped - basically the pieces just had to be polished a little and then put together. In looking at kits that needed to be drilled and tapped, etc. - the extra money for a finished lock was well worth my not having to mess with it.
The Siler Mountain Lock is a great lock. It is cut out for a 1/2" drum so remember that when you order your drum. If the barrel supplier can fit the breechplug and the drum, you may want to have him do it. Again, it depends on your metal working skills. Remember that you are going to install a 1/2" drum on a 3/4" barrel so you probably want to go with a 1/2" drum with 5/16" threads - just a suggestion.
A longer 42" barrel will work fine on a half-stock rifle - in fact, there were many original half-stock Kentucky style rifles. I had several southern made half stocks in my collection at one time - one even had a 46" tapered smoothbore 45 cal barrel on it. Try to find photos of original half-stocks and study them. If you are going to use a half-stock blank, you want to get the proportions right in regards to the length of the forestock with the length of the barrel.
If you are going to make a half stock - another consideration is how your are going to do the ramrod. In other words, are you going to solder your ramrod thimbles directly to the bottom flat of the barrel (which was often done) or are you going to use a "rib" on the bottom of the barrel. You need to know this so you can lay out your ramrod hole in your stock. Talk with the supplier of your blank and see what they sugggest and what they can do - can they drill the hole for you and space it for a barrel rib or can they can they router a ramrod channel in the forearm so you can use a ramrod where the thimbles are attached to the bottom flat? Are you going to use an entry thimble and end cap on your half stock or are you going to pour your end cap out of pewter? Some of these things may depend upon what your skill levels are, what tools you have available, what jigs you can make to help layout and guide a ramrod drill if you are going to drill the forestock, etc.
I'd suggest that you take a piece of wrapping paper and actually lay out and draw your rifle based on the parts you are selecting. This is helpful in laying out your construction steps that you'll need to do and the order in which you do them. You will have to decide how your are going to attach the barrel to the forestock - i.e. are you going to pin the barrel to the stock or are you going to use a key. You will have to attach a lug to the bottom of the barrel and it will depend on which method you are going to use what lug you purchase.
A lot of this may sound difficult but it really isn't. If you have some basic metal and woodworking skills, you'll do fine. Keep in mind that some of the work you do may require some tools that you just can't buy off the shelf. An example is in regards to fitting a barrel key to hold the barrel to the stock. This requires a rectangular slot be cut throught the forearm that the key will slid in to, through the lug and to the other side thus holding the barrel into the channel. On one rifle I was making, I needed to cut this slot perfectly as I was not going to use inletted plates on each side. I didn't have a chisel small enough so as I looked around my shop, I spotted a box of "pole barn spikes" that were six inches in length. I took one and finally gournd a chisel on the end that was the same thickness as my material that I was making the barrel key from. I don't know the hardness of the spike but it made a perfect chisel and it kept a fine edge when I honed it. It worked great.
Cherry will make a nice stock and is what I refer to as sort of a "medium" hardness. i.e. it is not as hard as hard maple and is usually fairly straight grained - each piece will vary though as with any wood. Keep your tools sharp. If this is your first rifle - don't get discouraged. You'll do just fine and when you're done, you'll have something that you can really be proud of.
As you construct your rifle, don't get upset if you have a "slip" or make a mistake - trust me, everybody has "oops". I not only built rifles but I was a cabinetmaker as well and I got involved with a lot of different projects. The secret to when you make a mistake is to figure out how to either cover it or work it in to the design. I've had the opportunity to examine a large number of original rifles and I have spotted mistakes on a lot of them - it might be a ramrod hole that went through the stock and a matching piece was inletted to cover it or a dovetail that was cut in to a barrel incorrectly and then filled.
You are going to be building a rifle that above all, you want to shoot. As Louie Sullivan said, "form follows function". Take your time and you'll end up with a nice shooting rifle. Of course then you'll have to make a powder horn and hunting pouch for it. Good luck!
