WiFi in rural location

You have more than 1 issue.

Issue 1: BT connection. Not sure what speeds you get/have but getting a reliable connection is key even if its only 5-10mbps. 

Issue 2. your home. You might need several avenues to get that working. Mesh networks are great but if you're relying on a wireless backhaul, your home might not be condusive to that. You might need to run ethernet, using moca adapters (ethernet over coax), or powerline adapters to get hardwired internet connections to your home. 

Issue 3. distance the barn is away. 200ft is to far for a simple router/extender. I'd look at a point to point system like netgear airbridge to get a connection to the barn from the home. 

issue 4. you probably don't want the customers on the same network you're doing transactions on. 

You'll probably need a mix of devices to get everything working. 

I know its expensive but I'd look at the business line of Orbi/mesh. That way you could seperate out the home and the business wireless. 

You'd also probably need a mix of devices to connect in the home to get good coverage. It might be cheaper and have more stability in the long run to run ethernet wires. but if its not possible, again look at a mix of powerline or moca adapters to get a hardwired connection. Then you can use the mesh in the home with a hardwired backhaul and use it for the barn but using a point to point system to connect the 2. 

Its not going to be a cheap option to do because of distance and the stone walls.