Earlier quoted context omitted.
Try http://www.tonido.com . It offers a remote drive feature. Will satisfy all your requirements listed here.
Hmm... interesting. Can you give user-specific logins to keep track of who's changed what?
I would suggest you to try the workspace application that comes with Tonido. Some NGOs are using it successfully to sync tasks, calendar, notes and files. The closest application that you can compare is microsoft groove.