> my Moodle instance has decided to go with RHEL
Yep, for an old-school Unix crowd an enterprise Linux is the wright thing.
> so I can become more familiar with the inner workings of Linux
This is a dilemma. Well packaged distributions tend to be "protective", i.e. the system will be well maintained if you play to their rules, say the configuration system. You'll be reluctant to try out bold things like editing config files.
I too began with Wallnut Creek RH and then quickly moved to Slackware. That was a gem. Slack is the most Unix-like distribution. The point is, you don't have to _stay_ with it. Your production server could be RHEL or Debian or whatever. For learing Slack is the best. Once that is done you can move to LFS http://www.linuxfromscratch.org/ if you feel like.