There's a lot of great advice on this topic. As a former CW reenactor and a current black powder collector and shooter who
browses the web regularly to find new "toys" (Henry's, Spencers, SSAs, Sharps, etc) there are PLENTY of places to get the guns, and they all basically come from the same importers/manufacturers so the only thing I really look at is the price of the gun from the respective vendors, shipping, price of ammo etc before making the purchase.
Tennessee alone has many places where you can buy these guns already, most notably Blockade Runner (between Nashville and Chatt) and Dixie Gun Works in West Tennessee.
Through there may be some retail traffic opportunities because of Chickamaugua/Chatt NMP, please do not confuse the traffic there to that of Gettysburg. Both battlefields are original to the "Big 5" national battlefields/military parks that were formed in the late 1800's but the traffic at Gettysburg and the potential profit there is much greater, even with the number of vendors there, because of the major cities and transportation hubs nearby (Harrisburg, Philly, Baltimore, DC, NYC, etc)
I would think the lack of similar place in the Chattanooga area MIGHT be to your advantage but the Civil War tourist culture is much different here based a lot upon the fact that you have to want to get to Ft. Donelson, Stones River, Franklin, Chattanooga, Resaca, etc. All are fairly easy get to but they are not, primary, destinations for the summer tourist like Gettysburg. Don't get me wrong, they get visitors that are there specifically to see the history (Carnton Mansion in Franklin averages 60,000 visitors per year) but I would guess that most of the seasonal traffic is people pulling off I-24, I-65 or I-75 on their way elsewhere. Gettysburg is an international destination.
If this is a passion of yours, I'd start web first. If interacting with customers and talking history is what you want a small retail location in Ft. Oglethorpe on GA 3 near the relic shop (I forget the name of it but it's in a strip mall type complex at a major intersection) might be the way to go. As noted above the area near the main entrance to the park and visitors center is a bit "sketchy" and I think even the McDonalds near the gate closed (at least it was shut down when I was there last) and if McDonalds closed that's a sign, and something you should research in depth, because they're practically recession proof especially when located at the entrance of a national park/battlefield.
You might also want to explore a downtown Chattanooga location - that area has seen extensive urban renewal and experiences a ton of traffic and tourism. While not near the CW action directly, it gets the visitors that Lookout Mountain, and other places simply don't.
I love to handle these guns: Enfields, Springfields, Sharps, Colts, etc and visit and talk with like-minded people and could hang out in a place as you envision for hours but I think I'm in the minority. To make money in the business now, in my opinion, you have to cater to the casual and impulse buyer who is moved by what they learned, saw, felt at the battlefield and simply HAS to own a piece of, or a reproduction of, history NOW...before the passion fades. Additionally, the stuff you'll have to sell to really make money to pay the rent and expenses like t-shirts, candy, books, tin cups, canteens, uniform parts (quality or otherwise) toy soldiers and other kid friendly stuff (typical Civil War disposable "junk" you find at most tourist shops, even ladies things like period apparel, etc). in 1994 you could find a Euroarms 3 band Enfield rifle musket everywhere for less than $400 - this same gun now is routinely around $600 or more. That, while very "affordable" compared to a Henry, Sharps or Spencer, is a lot of money for a casual sale to someone who walked out of a battlefield visitor's center earlier in the day.
If it were 20 years ago and the reenacting hobby was booming post the Ken Burns PBS Civil War series I'd tell you to go for it NOW but as we're almost half way through the 150th Anniv celebration of the CW, you might be on the backside of the popularity before you really get up and running.
I'll stop rambling...you just need to decide if you're going to do it for the passion and fun of history or as a money making venture and follow your heart.
Best of luck in your research and decision and if you open, please let us know. I'll make the 2 hour drive from Franklin!!