I'm working on the exact same thing.
I've tried a lot of CMS systems (it also needs to be free for me) and so far I like PHPNuke the most (
www.phpnuke.org). I've read some comments that this one is a bit buggy and insecure, but those comments are all related to version 4.x. I've tried the new version 5 yesterday and really liked it.
It's easy to customize and all. I've got a few other ones lying around but I can't test those until I got .JSP files working on my server.
Hope this helps and I welcome all further comments on CMS systems as I'm also still searching.
Michiel