Dear Sir,
I completely understand your question and the financial issues. First, having owned Discriminating General/Military Heritage (1) and Middlesex (2) and Loyalists Arms (4) I would ONLY go with Loyalist. The quality and customer service are superior, and the Higgins are wonderful people that will treat you right. I can't say enough good things about them.
Second, I think there is a difference between "common" and 'likely possible'. I have some 'modern' guns that will be 100 years old in 2028, that are still great, usable firearms. However, if a person was going to war, hunting and using a firearm for protection, how long would they commonly last? We KNOW there were flintlocks, Besses, 1816 and 1803 Rifles (some that were converted) in Southern States). Did folks want Enfields or Springfields? Sure, but look at the book Butternut Brown and Cadet Grey for great descriptions of clothes and weapons here in the South. In Missouri, MANY folks were using squirrel rifles and shotguns.
As to reenacting, the time periods you are interested in are all flint eras. In many WBtS reenactments, they do not want flints used. I would say if you are dealing with normal reenactors (not the folks that will starve themselves to get the emaciated look) most will be accepting of an older style that will be used in later periods. Obviously don't by a matchlock, but you are probably safe with anything 1760 on. That said, check with any of the local groups you would be dealing with to see if they have any other requirements.
Respectfully,
David