[Skunkworks] Cyber Cafe management system [Linux]

Am looking for a linux (ubuntu) cyber cafe management system for a cafe migrating from microsoft to linux. Just in case am not clear, the system should help the cafe admin to easily bill their customers after they are done surfing. It should lock all workstations when not in use and start timer/billing once logged in. Your pointers to the right applications (open source) are greatly welcome.