we currently have a froum (that no one really uses) to post relevant news onto. this is great in the fact i can restrict to different user groups who reads what. we are now looking to convert to a blog (as its a lighter load) but want to retain a multi level access system. eg public, level 1, level 2, author/admin
wordpress comes with built in user permissions. some people can do the whole lot, some people can make comments, some people can make posts, some people can post and edit, some people can only post once it's been moderated etc...
and it's open code as well, so if you want to change the permission settings then you can do
I've done this with Drupal sites. You make certain sections viewable only to certain groups. That's built in to the package. No plug-ins or add-ons required. Look at portal packages instead of blogs for this sort of functionality. You want to control a community, not simply broadcast like a blog.