Converting static site to Drupal 7 - How to set-up department sites?
I think multisite is not the optimal solution for you. With a multisite installation, you will end up with completely separate sites that have their own content, categories, users, settings, user files etc. etc. In my experience, that is often too much separation for a site like yours.

For instance: With multisite, if one person needs to get access to two or more departments, you need to create a user account for him in each site. He will also need to login in every site. Another example: With multisite, it's not easy to take content from a department site and promote it to the main home page.

In your case, I would look into a solution built with Workbench and Workbench Access, or perhaps Domain Access. With those modules, you will end up with a system that is technically a single site, but where Drupal is configured to split the site into sections and control user permissions on a per-section basis.