How much customizability are you actually looking for? If you want it to be
highly customizable, and aren't afraid to get your hands dirty you should seriously considering building your own. I'd recommend Django which would minimize the amount of effort that would need to go into building a CMS, and comes with a user management system built-in
plus (and this is my favorite part) will automatically build an admin interface for the site which saves some
serious time in development.
Or, if you didn't want to have to build it yourself you could hire me to do it.
Without knowing exactly what you have and what you want to do with it it's hard to give a good recommendation, but I'm always in favor of starting at the lowest reasonable level of complexity and building it up yourself. For reference, my business site (
dy/dx tech) was built with Django, so basically all the content that you see is stored in the database and easily modifiable through the admin interface Django automatically generated without any work on my part and protected with the built-in user management system. Took me about a day to put together the whole site (not counting the design which I had already done when I originally developed the site as static pages). Having switched it to Django it will be
much easier for me to update it as well as add new features (several of which are currently in development).