Looking for advice on wifi for restaurant

[BT5150]

I am opening a new bar/restaurant and I'd like to offer free wifi for my customers. I will also have a POS (cash register) system that will use the same DSL line for credit card processing, so security is a big concern. The POS system would directly connect to the DSL modem, which has 4 ports, it would not connect wirelessly. I have contacted 2 different networking companies in my area and they came back with somewhat different answers. The first company recommended using 2 separate wireless routers which they said would be set up on separate LAN's. This would allow me to have my own wireless network to use my computer on and the second one would be for the customer's use. The second company recommended using a router called "watchguard" which is set up for this kind of use. They didn't mention anything about separate LAN's. Does anyone have any experience in this type of setup and what would you recommend? Thanks!