I recently set up a FreeNAS server on an old Pentium 4 box and that was really very easy. I think it's mainly set up to be a file server, but it has plugins, etc. Might be worth checking out. It's based on FreeBSD, however.
Superb Mini Server is a Slackware-based server distro that uses webmin for administering the system. I've never used it, but I use Slackware and I've used webmin. Slackware is rock solid and it looks like you can configure it for whatever you need.