I recently came across an OSS project called "Epiware." Here a link
to the site. I looked it over and decided to install it on a Linux server and give it a whirl. It is a very capable DMS but I didn't find the "Desktop" interface intuitive or particularly easy to use. I figured out how to do what we (in my district) might need to do, but I know that many of the administrative assistants will find it difficult to use.
Take a look and let me know what you think about the possibility that something like Epiware or a similar product might hold in meeting your DMS needs.