Beware - this is just my opinion. Other may have different thoughts.
Some of those made by Pedersoli, and maybe Lyman are okay, and may be a decent choice for a newby just getting started with flintlocks, but I wouldn't bother with any of the other mass produced ones.
In my opinion, the important thing is to get a good one. Too many of the cheaper ones (under $1000.) do not have a good, hard frizzen and don't spark well.
Companies such as Track Of The Wolf, Tennessee Valley Manufacturing, Caywood, and The Log Cabin shop all sell custom or semi-custom muzzleloaders as complete ready to use, as kits, or "in the white" where all you have to do is to put the finish on them yourself. They all have websites that you can look up.
As for shooting and maintenance, they are really pretty simple. Load just like a caplock, except instead of a cap on a nipple you have a pan. Black powder, of course. The synthetics do not work well in a flintlock. You do not need 4F priming power, either, although it is certainly okay if you want to bother with it. I use 3F for the main charge in all my guns from .45 to .62 caliber and prime with the same powder. Some say 2F works better in the larger calibers, and maybe it does, but 3F works well for me.
Powder first. Then a patched round ball, unless you're using a smoothbore and shooting small shot. That's a whole 'nother ball game.
Just a little powder at the outer edge of the pan to prime. Don't fill the pan with powder.
Keep the gun clean. Run a toothpick or small wire into the touch hole from time to time to make sure it is clear and the fire from the powder in the pan has a clear path to reach the main charge.
Clean it like you do your caplocks, but be careful that dirty cleaning fluid doesn't run out the touch hole and make lines down the stock.
Mainly - have fun!
