SlackWare, or RedHat 7.3 (not RedHat 8.0)
RedHat's up2date is nice, and it keeps things secure as far as new vunerabilities and bugs go. Unless you want to do this manually.
If you want to maintain it yourself you should use SlackWare. RedHat can be used as well but is not as well suited for someone who wants to build everything like Slack is.
--mailserver - I like postfix..
if you have external clients you need to use tls to have secure logins. (no open relay) Use secure pop, or secure imap for users to retrieve mail over a secure connection. (no unsecure logins)
--Webserver - apache
--DNS - if you need it
--DHCP - no problem
--Database - good choice
--Admin Tool - My opinion is do not trust the gui's to do it for you.
--Filesystem - I use ext3, no problem for years.
--Firewall - iptables, using a solid set of rules to allow what you need and block all others..
--FTP Server - sounds good, I would consider sftp (ssh) if you are doing anything other than anonymous ftp. No users logging in using passwords over ftp.