I found this PDF report comparing WordPress, Joomla, Drupal and Plone quite helpful when I was asking myself the same question recently. Nice comments with a focus on non-profits - find the comparison summary/recommendation on pp 13-14.
Definitely read the report because it talks about the strengths/issues of each package - given your prioritized list of features, I'd recommend Joomla. I think you can do what you want with reasonable effort and good recent modules like DocMan. WordPress is awesome for a blogging site, but lacks a number (2,3,4,9) of your requirements, and though Drupal is powerful and flexible, it definitely has a steep learning curve. Plone even more so.
Good luck!