connecting two networks

Hello,

Here is the situation. This group I'm helping has a dsl line and four networked computers in an office. They want a hotspot in a coffee house about 30 feet away. I'm wondering how to keep the office network seperate from the coffee house network. I was thinking of having a dedicated computer in the coffee house basement running the hotspot software and a linksys wireless G router upstairs for three terminals and the hotspot area. They have a few computers that were donated and 2 wireless routers, the other is a D-Link 514. I'm thinking of setting it up similiar to this diagram and using this software URL I'm wondering what else I need to do or what I can read up on to do this the best way.

Thanks