Star topology using Cat5 should be fine, Use switches not hubs, the servers you need depends on what the clients will need to do (will they need e-mail, will there be network printers etc.). Anti-virus is a difficult one to answer as there are many products out there, we use McAfee VirusScan which is deployed and managed by ePolicy Orchestrator. You will also need a firewall and some kind of virus/traffic scanning software at the gateway to scan e-mails/web traffic etc. for nasties. Hope this helps a bit.