I have a pretty similar setup at home, without the anti-virus which is done locally on the PCs, my server is an old Duron 700 with some spare harddisks in (280Gb), running CentOS 4.2, samba for the network file storage and an iptables firewall. Does DHCP aswell.
Setup is straightforward, usual graphical installer and pick the packages you want, iptables firewall examples are all over the web and its run as part of the startup process. Simple Samba configuration for a network share is easy enough, and you can do much more advanced things aswell.
Network configuration is easy once you have figured out the topology - I have the external router as one external IP and the server as another so I can connect to it remotely, but your ISP might only give you one IP for the router.
I picked CentOS as I have a bit of RedHat experience, but virtually all distros will be able to run what you need
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