Want to setup an intranet for a small business setup.

Current Setup

• 28 computers running Windows ( few older pc's run Windows Xp but most run Windows 7)

• Spare Dell Pentium 3 which can run as a server.

• 6 switches spare NIC's and lots of lan cable available for networking.

• 3 Independent Internet connections

Currently we have 3 independent networks which share internet connections, each network uses a different internet connection. Current network is setup solely to share the internet connection.

What I need to achieve in this intranet

• Setup one common network.

• Instant file transfer via local network (maybe setup a file server?)

• Local text and voice messenger software

• Bridge the 3 internet connections and route all the internet connections from the main server

• Ability to allow or deny internet access to any computer on the network.

• Remote access from the main server to the client pc's on the network to debug software issues

What operating system should I use on the main server? Do I need a hardware firewall?

Any setup guides / resources or how-to's on how I can achieve the above requirements.