The fact you have a router does no affect your requirements for a AV. So stick with whatever you have now, if you dont have one get one, consult the AV Poll for more information.
In terms of firewalling the PCs on the LAN you may want to continue to run a simple firewall on the workstations to allow you to control applications internet access.
A broadband (NAT based) router will stop unsolicited incoming traffic but thats it. Anything is allowed out which is why you may want to keep control of what applications can communicate with the internet.