My goal is to have a flintlock muzzleloader for plinking and target shooting. I also like to learn more than just how to load and shoot. I'm fascinated by mechanical things. It would be interesting to learn what makes them tick. Hence the desire for a kit build. I own pistols and revolvers and looking for something different to enjoy from the sports of shooting.
Reading is a good first place to start since you state you have no prior muzzleloading experience. Go to the gunbuilders forum here and look at Dave Person's exceptional tutorials. Dave not only shows what he does, but explains why as well. Take careful note of the architecture. The Gunsmith of Grenville County is a comprehsive guide to building longrifles. You wouldn't need all this info to build a kit, but if you really want to learn, it's a good book to have on hand.
https://www.amazon.com/Gunsmith-Grenville-County-Building-Longrifle/dp/0985796901
For plinking and target shooting I would choose a .40 to .45 caliber rifle. These are both really nice calibers to shoot with less powder and lead. Certainly nothing wrong with a .50, just more than you need for plinking and targets. In a flintlock I'd get a quality lock. A swamped barrel is, IMO, a very nice "upgrade" for weight and balance.
I don't know that you'll get a good quality kit for $800 or less. I think it will be $1000 and up. Check out Jim Kibler, Jim Chambers flintlocks, Track of the Wolf, Pecatonica River Long Rifle Supply, and others. Kiblers are the easiest and high quality. Chambers take more work and are generally high quality, others require more work in wood shaping, lock cleanup and tuning, etc.
Low cost options are available, as already suggested. These kits will produce a gun that shoots. Only you can decide what your cost/quality/historical appearance balance is.