Ideal setup for 20 computer NT network

What is the ideal setup for a company with 20 users (computers)with a cisco 800 router for e-mail and internet connection. We must have a PDC with DHCP and DNS for the router. Raid 1 is suggested as essential and a bacup tape drive. with this equipment installed do we need a BDC and should we use a File server as well. Your thouts on this would be much appreciated.