Windows Server 2003

I am installing a windows 2003 server.

If you are already using POP3 email accounts with a hosted service because you don’t have an exchange server why would you use this option within server 2003?

If you set up as a domain( we are currently a workgroup) do you have to use the server for DNS and DHCP or can you continue to use the router? Lastly if you use the server for these services how will it handle file sharing as far as perfromance….15 users 2 servers(1 is a 2000 server set up as an application server in a workgroup)

Thanks.