There is no required SP or patches for BES to be on Windows 2003. SP1 never hurts to have but is not necessary.
Make sure to read the install guide for BES Express from RIMs BlackBerry Technical Solution Center
. It's under Product Documentation, if you follow the pre-install steps you should have a fairly easy install and the user setup should also work fine.
If you have any questions about any part of the pre-install or port install let us know.