Have you considered using Google Apps for Your Domain and just using the Google Docs platform? This can be implemented "on your site and, at this point in time, is arguably the “best of the breed”.
You might also consider who really wants to “edit” in a web environment - often it is preferable to simply allow them to upload their edited docs, or .pdfs, and let them use their own tools. It’s easier on the server, the user can use the tools that they like, and your life would be much less complicated.
Just my two cents worth …