CMS to administer multiple sites from one central admin

We need a solution that will allow us to administer many sites from one central CMS. We will have between 10 and 50 sites on individual domains; some content will be duplicated over all the sites however some of the content will be unique to the individual site. We need to be able to make a change in the duplicated content and have that change be reflected across all the sites but be able to make changes in the site specific content without affecting any but the desired site. We will need to be able to affect the look/design of the sites through CSS/skin files so that we can easily change layouts etc. We will need to be able to easily add new sites on new domains, and have all the duplicate content copy over straight away with just space to add site-specific content. We have been looking at DotNetNuke, Wordpress & Wordpress MU, Moveable Type, Drupal and Joomla. They all look like they might be able to do it but without installing them and trying it out we can’t see which if any of them are going to allow us to duplicate content across the sites. It’s no good if to change a banner or a link we have to make the same change 50 times, we need to be able to make it once and have all sites update. Similarly site-specific textural content has to be unique to each site. We need someone to say either “yes you can do it with the ‘X’ CMS and we can install it and configure it to look how you want and behave as you want.” Or “No it can’t be done with any off the shelf CMS, you need a custom system and we can develop it for you.” I have attached a rough sketch to try and illustrate what we are trying to do if it’s not clear above. PLEASE BE CLEAR IN YOUR RESPONSE HOW YOU PROPOSE TO SOLVE THIS - by which I mean if you think you can do it with DotNetNuke, please explain how it is possible with DotNetNuke. If you feel the stated budget is out please still bid and we can discuss what you think is involved. Many thanks