Content Managment System Needed

I need a Content Management System that meets certain requirements for an RPG project. I can't find one that has everything I need or even one that has almost everything I need and I can wedge in some other components to cover the rest. So much to my chagrin it looks like I am going to have spend a whole lot of time writing a CMS when I really don't want to. Just for the hope that someone might be aware of something here are the requirements.

Absolutely Positively Must Have

• Linux / Apache / MySQL 4.1+ / PHP 4.3+ Based
• Search engine friendly URLs - URL rewriting if needed - inline links must have friendly URLs as well - no overtly dynamic URLs with querystrings and the like.
• Must have a category/tagging system similar to wordpress, must support sub-categories.
• Blog / News - A multi user blog is preferable to a news system and must work with categories/tags in a technorati friendly way.
• RSS - Must support all major feed types, RSS/Atom etc.
• User registration system - a simple user registration system that doesn't require much to get started.
• Articles - Managed article system which must also work with categories/tags.
• Downloads - Managed download system which must also work with categories.
• Must be able to run multiples sites (Site A, Site B, Site C) but only utilize a single user base.
• Must also be templated using XHTML and CSS - should not use tables for layout.
• Built in wiki and/or MediaWiki integration including users.
• A simple online store what again works with categories/tags
• Modular - must be able to add additional features/extensions in a modular fashion

Nice to have - not required

• Ajax functionality

I know it's a tall order but that's what I need (in brief.) If anyone knows of any CMS that comes even close to this please let me know. If it's open source I'd be willing to diverge a new system from the old.