Best tools to build a collaborative project site using only open-source Apache/Mysql/PHP tools

I am in the process of building a project site for my current project at work and I need help with the best of breed tools for this site. The site will be used by the project team (about 12 members) for file storage and collaboration.

It need not be designed for heavy traffic but should…
1. Be easy to use
2. Be easy to upload/download multiple files to/from
3. Be easy to tag or categorize files
4. Have the ability to create custom folder structures
5. Use existing open-source CMS or collaboration tools

It would be nice if it…
1. Were easily themeable
2. Based on any combo of the following systems:
-b2evolution
-Nucleus
-WordPress
-Drupal
-Geeklog
-Joomla
-Mambo Open Source
-PHP-Nuke
-phpWCMS
-phpWebSite
-Post-Nuke
-Siteframe
-Xoops
-phpBB2
-SMF
-4Images Gallery
-Coppermine Photo Gallery
-Gallery
-PHProjekt
-TikiWiki
-PhpWiki

It needs to be able to handle uploads of up to 20MB files with unlimited file types.

Please let me know what you think is the best framework to use for this and if you can suggest any similar sites for me to check out or literature to read I would be thankful!