Hi!

Seems pretty vague requirements. There is probably a way to make that happen with MediaWiki.

Otherwise, feels like any of the more enterprise wikis will do (Twiki/FosWiki, XWiki, Tiki, Confluence, MindTouch, etc.)  You could pick according to your preferred technology. Eg. if you are a Java shop, check XWiki or Confluence.

I'll comment for Tiki, and I suggest that people familiar with other engines describe how they would do it.

> So what I need now is a means to administrate different user groups: The customers should be able to access only a part of the wiki. Furthermore there should be a workflow in order to make the articles "customer compatible". E.g. someone must first have look on the contents and reworks them, if this is necessary.

So I would create 2 categories:
* Staff (which contains all the pages NOT visible by customers)
* Customers (which contains all the pages visible by customers)

And 3 groups:
* Writers can make edits, but these edits can't be seen by customers
* Reviewers approve changes made by writers
* Customers can read (and perhaps comment) but not edit

In Tiki, this would be done with
* http://doc.tiki.org/Permissions
* http://doc.tiki.org/Flagged+Revisions
* http://doc.tiki.org/Category

> The articles should also reflect our corporate idendity and design style guides.
Tiki makes massive use of CSS, and uses the Smarty Template Engine: http://www.smarty.net/ so you can easily change the look & feel: http://themes.tiki.org/

If you need to import some data from MediaWiki:
http://doc.tiki.org/MediaWiki+Importer

Best regards,

M ;-)