DjamesF, posting the area you live in would aid us in suggesting groups to contact.
When I started in BP shooting my first two were cap&ball revolvers which I dove head first into using for cowboy action shooting. It went relatively trouble free thanks to lots of reading off the BP sites such as this forum and you tube vids.
When I wanted to go with a BP long gun I wanted to go as different as I was able. So I went with a flint lock instead of a cap lock. I have no regrets that I went that route but the others here are right and it IS a big hill with a steep learning curve. Shooting with the BP group at my local club proved well worth it. Aside from the fact that I enjoy the socializing I was able to get lots of great hints from the other "rock lock" shooters. So if you don't mind a LITTLE bit of potential frustration early on having that big flash go off right in front of your nose really is a hoot!
What you need to know first off is that the lock on a flint lock is everything. If the frizzen is not hard enough it won't spark reliably. If the frizzen does not ride on the tension spring correctly it will give a reduced shower of sparks or will result in reduced number of firings from the flints. Also flint locks pretty much require real black powder where cap locks can get away with the modern substitutes used for muzzle loading. Namely Pyrodex and Triple 7. These two powders being a lot tougher to ignite.
I do not feel that it's good economics to buy a cheap flint lock only to be required to replace the lock. Good lock works are not cheap. You may as well just buy a more expensive flint lock right off the bat.
A fair number of the smoothbore fellows in the local group run muskets purchased from Loyalist Arms. They have nothing but good things to say about the outfit. But they all know that they are buying a gun that needs a lot of tweaking and fettling to turn it into a reliable firearm. More than one of them found that they needed to re-harden the frizzen. And more than one needed other detail fitting to make these guns all fit together well and shoot reliably. That isn't to say that you can't just drill the touch hole and have a good shooter. But don't rely on that being a certainty.
There's also the economics of a smaller caliber in terms of lead and powder used. The bigger bore smooth bores use cast balls that can be used to make two or three times as many .45Cal balls. And roughly twice to three times the powder to shoot the larger size bores as well. So there's much to be said for a smaller .45 or .50 Cal rifled barrel long gun. And it's certainly been my choice.
If you plan on hunting you'll want to stick with a .50 or .54 for the added take down power. Otherwise a .45 makes for a nice sport target shooting gun.
Traditions is a fairly well known and respected brand for cap locks. There are a lot of them in my black powder group. But there isn't a lot of love for the Traditions flint lock rifles. I seem to recall that it was related to frizzens that were too soft and didn't spark adequately.
If you should happen to come across an Armi Sport flint lock or cap lock you should not walk away.... I'd suggest you run instead. Run fast. I bought one way back when I started and only shot it about a half dozen times before the lock work got "sticky". Opening it up and comparing it to pictures and write ups explained why. I won't go into details but they took a short cut and didn't chose to use a part which is considered a basic necessity in all other locks. All to save a few pennies.
If you came across a Lyman Great Plains Rifle for a reasonable price I'd say that would not be a bad place to start. I've been very happy with mine. It was hard on flints at first until I learned a trick from my recently acquired custom flintlock about how the frizzen should move. A small tweak and now I'm getting up around 25 shots before the flint edge needs to be knapped sharp again and then I get another dozen or more before it rounds over again and is too short to use.
And yes. at some point if you get a flintlock you will find that you need to buy your flints from somewhere like Track of the Wolff by the dozen. They don't last forever. With a little fixing of the edge about 3 dozen firings is about all you can expect.