I wouldn't worry too much about allowing use of your Internet connection. I would recommend segregating your internal network and creating a DMZ, which is where the hotspot would live.
Another thing to consider is if your Internet connection is shared between the hotspot and your production processes (such as Point of Sale) rely on it, you might risk affecting them if someone to saturate the link. Implementing a Quality of Service (QoS) protocol could help eliminate the risk here.
If my two earlier points are outside of your technical ability, it might make more sense to dedicate an Internet connection as opposed to risking exposure of your point of sale system.
Another item of note is that commercial or shared use of your Internet connection is likely against the Terms of Service of most DSL and cable connections. This is something to be aware of in regards to how you utilize your existing Internet connection.
In regards to implementation, if I were to do it, I would probably leave it open. I see no issue with making it public and free. If you are looking to capitalize on it, you will probably have some usage management.
The quickest and easily solution would be to use a router running DD-WRT, which is something I've had incredible successful with. It also comes with some hotspot solutions, which would address some of your questions automatically as well as make implemention of QoS simpler.
All in all, you'd benefit from a Google search. There is already loads of information available on this topic.
Chillispot on DD-WRT