I can't help on the Mail or DNS servers much, but I would suggest Apache for a webserver. It's very flexible and is included in Red Hat 9. In most cases, after you install Red Hat, Apache will run automatically at boot, with no configuration needed.
Basically, what you would need for a full-blown server are these programs:
- Apache (web server)
- Bind (DNS server)
- .... and I can't say the name of a mail server off the top of my head, but it's listed in the server section of the Red Hat installation (or add/remove programs).
That's my little tid-bit. Sorry I couldn't say much more. I'm not a server guru by any means (yet...