Best solution for providing an open and secure network

You can have one or the other but not both.

If you open the system then everyone who wants to has access to the Servers and any Files that reside on them.

Places like Starbucks [b]Do Not[/b] allow people who use their WiFi access to internal Company Files they just provide a Open WiFi Access Point with a limited ability to search the net.

Col

That's a very open-ended question, but normally you would create a new SSID for guest/untrusted access and use VLANs to keep it separate. You'd generally put a DHCP server on this VLAN (or if you're not big enough to justify it, allow DHCP traffic to pass to that VLAN) and would also open port 80. There are plenty of WiFi devices around that support all the things you typically need to consider - VLAN tagging, enforcing bandwidth and total download limits, blocking things like P2P and porn, requiring acceptance of terms of service before being allowed to connect etc. Also, if you're talking about supporting phones and tablets check out MDM tools. Meraki have a good free one that you can play with to see the sort of features MDM tools support.

You wouldn't allow access to the database from the guest WiFi network. Set up a terminal server or some other way to authenticate first (eg. make it web based and require credentials, or use VPN).