Speccing a Windows 2003 Server, how much do I need?

This server needs only to support a 20-computer Local domain, and another 10 users from another site connected via a terminal server which is VPN’d through a VPN device/Firewall.

This SBS Server will need to run Exchange, and SQLServer for a piece of software which is used constantly, but only has a peak SQL Transaction rate of not more than 40 queries a minute for all of the machines.

Exchange will be being used typically.
With no Webmail functionality, but with IMAP for remote “dial-up” users, and exchange for the 5 remote users connected via the Terminal Server.

Looking at the spec and considering reasonable costs.

What would the minimum required to provide relatively swift going for all users?

Many thanks in advance.