Maple was quite common on Southern Mountain rifles, as was Walnut. Either is a proper choice. Furniture was predominately iron. Southern Mountain rifles had English locks. So if the kit you're considering has a Germanic lock, like a Siler, it's not what was used. I've never seen a photo of, or read of, a Southern Mountain gun that didn't have an English lock. They also had swamped barrels, which most kits do not have. Swamped barrels make all the difference in the world when it comes to handling and shooting.
I'm not very knowledgeable when it comes to all of the kits available, but I do know that many of them come with straight barrels and Germanic locks.
Southern Mountain rifles are dear to my heart; and mostly what I build. I wouldn't call myself an expert, but I do know a thing or two about them. And I tend to be pretty opinionated. If I didn't roll my own, I would be shooting a Kibler Southern Mountain rifle. The extra money is well worth it when it comes to the finished product.
It's no small investment in money and time. The most important thing is having a gun you're happy with years from now.