Drupal 6 as an intranet solution

Im considering using drupal as our intranet solution.

But it must be easy and intuitive for its users to write stories/forum topics/posts/articles etc, and choose both users and groups to which it is assigned.

Im unsure if i sould use nodeaccess, node_access og ACL+Forum and Content Access.

Our organization are kind of two hierarchies. One that all employees are members of. With a board of directors, and multiple suborg. An employee kan be member for more than one suborg. or none.

An employee can also be a member of a second hierarchy. This is based on the level of "safety" education. The board does not control this hierarchy.

see a picture at http://aaholm.com/hierarchy.jpg