It really depends on the setup the office will want. If they will be running several computers from the AC database then they may want a server to have extended capabilities for managing and maintaining a larger number of computers. For those that want a small, 1 to 5 computer setup, they could use one of the existing machines as a server with cheap RAM and NIC upgrades. Functionality is all about asking what one wants vs what one can afford. Same with security. In the office do they need a domain setup or are they comfortable with a workgroup setup? This has a lot of variables that would have to be addressed to provide real concrete numbers.
