I built myself a .32 flinter years ago and after the first day of hunting with it in temps that never got above 35°F, I went home and built myself a ball dispenser. Yes, the .32 is a great squirrel rifle and I've taken grouse, bunnies and woodchucks with it too but I still prefer a .36 - the slightly heavier ball gives advantage in power if needed and maintains a little better in crosswinds too.
I assembled two kits for myself and a dozen+ for others, while it seems that all the hard work is done for you, I can assure you that most kit guns I have worked with has taken far more time than building one from scratch if you want it to look right and even then, most still have problem areas that neither look right nor please me. Gaps around the pre-carved inlets, inlets cut too deep, screw/pin holes that don't line up... nothing annoys me more than having to repair a gun before it's even built - it's like going to the gunshop and looking at a modern suppository gun sporting a $850+ price tag and immediately seeing that the barrel channel isn't centered, gaps around the trigger inlet, buttpad offset and machine-cut checkering panels that aren't even close to being in the same spot from side to side... how can anyone, big company or not, allow that nasty ugly crap to get out the door let alone be promoting it? Sorry, didn't mean to go off on a rant, I just believe that if you're not going to do something right, just don't do it at all.
I start with a plank myself but if you're planning on a full-stock, a pre-shaped blank wouldn't be a bad thing if you don't want to make the tooling for cutting the RR channel and drilling the RR hole - however, a pre-carved blank also leaves you without flat sides so you need to work from the top flat of the barrel for the horizontal reference, not a big deal but you have to pay attention to what you're doing. If the tang area has been cut just a little too low and you can't lower the barrel inlet enough, you have to live with nasty look of the iron sticking above the wood. Once the barrel is in, the lock comes next, again, not difficult but the inlet has to be placed correctly and accurately - another reason I don't like kits is because you have to live with where ever someone else decided the lock should go and that simple fact alone can make or break the ignition speed as well as the overall look and quality of the build. If the touch hole isn't already drilled in the bbl, usually you can adjust it enough to match the lock position to counter some of the ignition issues but if the lock mortise is too far off, you likely won't be able to get a standard liner in and you'll have to make a special one or do without. If you're going to do an relief carving, you may want to leave the wood around the mortises and tang higher in elevation than the finished height until you get the carving done, can't usually do that with a pre-carve.
As far as getting individual parts and building what you want, there are no issues with that unless you are trying to exactly duplicate a specific rifle built by someone else in the past. If you are building it solely for your own happiness, then you can use whatever parts you like and put them together the way you want. If you're copying say a Lancaster or Jaeger, then there are certain parts you must use for it to be considered historically correct but there are no "rules" that say you have to build historically correct and I will argue the point that there were quite a many "historically correct" guns assembled from whatever parts people could find and set into whatever piece of suitable wood was handy - the only thing you have to watch is dates of origination if you wanted to stay within a specific time period.
Relief carving
Combination incise & relief