Setting up 10.4 server for web and mail only

I’ve got an xserve which has been working as a web server only for some time now, and working just fine.
I need to add mail server to it.
All of this was previously done on ASIP, and with the small number of users I will *NOT* be migrating.
Our ISP does our DNS for us and has created MX record for the new machine’s IP address <-> the mail server.
It is a standalone server, the only one on campus.
It has its static address.
The router does DHCP.
The router (and only the router) does firewall.
No FTP no AFP no client management. Just web and mail.
Do I need to do Open Directory?
Do I need to run DNS on this server?
What is the minimal /critical path to add mail service for a initial dozen users.
Thanks.
I have Schoun’s visual quickpro guide at the ready.