There are tons of things to learn when building a gun. I'd first decide what you want to learn. I'll give a BACKWARDS list of some of the things you probably want to learn while building a gun.
Finishing: you are a woodworker, so finishing wood should be no problem for you. Finishing the metal would be new. Filing, polishing, browning, bluing, or leaving it bright, would be new, something to learn on a first gun. You'd learn this whether you do a kit or scratch-build.
Inletting- some kits, the parts practically snap into the inlets. Others, there's a smidgeon of wood to remove. I find it harder to remove a little wood from an inlet that is almost the right size than to inlet a lock into untouched wood. The pieces want to wander more in a pre-inlet mortise. Still, it's one thing inletting a buttplate onto a pre-cut mortise, and another entirely, sawing a blank to length and starting there. I'd find using a non-inlet precarve the most difficult of all, because almost nothing is square anymore. It would be very easy to get things slanted a little bit. A blank can be kept square, parts inlet, then shaped.
Shaping: If you start with a kit, you are just shaping the gun very subtly. You'll learn how to use files and rasps and scrapers (which you already know) but not saws, planes, drawknives, spokeshaves, gouges, etc, to really shape a stock.
Design: If a kit, you have nothing to do with design. Somebody else has made all the parts choices and has set the architecture of the gun for you. You'll not learn how to position a lock in relation to the barrel, triggers in relation to the lock, buttplate in relation to the triggers, and how to lay out the buttstock profile in accordance with your preferences, while keeping the pull, drop, castoff, where you want it. There are many more things you'll not learn even with a pre-carved stock, or with a blank with the barrel inlet and ramrod hole done.
Making parts: Someday you may want to learn to make parts. I built a couple of half-stock kits, then felt ready to scratch-build. My first longrifle from a blank, I made the sights, underlugs, nosecap, sideplate, toeplate, trigger, trigger plate, patchbox, and patchbox release mechanism.
So I'd advise you decide what chalenges you want to try right now, then going at it step-wise. Maybe make your first gun a kit, then take on something more ambitious.
I also recommend a simple gun for the first build. A musket or northwest gun, fusil ordinaire, or an unadorned longrifle is a good place to start.