[stealthn]

I would bring in a firm to provide options and quotes.
From a business perspective there are a lot of unanswered questions: who is going to pay for the gear? What about liability between companies? What if one company uses/requires the majority of the bandwidth? Who will monitor and manage the network?

A simple setup would be a good firewall with multi-lan capability, a switch or switches for separation of traffic (a vlan is not really security solution) and access points.

My two cents