Yeah buy a file server and office. Seriously.
Managing a file server, even if it's just a static workstation on a windows network is easy from security to backup - it's all guided configuration.
Seriously - everything that google apps does can be done on offline MS office and a cheap hosted exhange account.
The web is shitty for serious productivity apps from experience.
Regarding open source, use openoffice instead.
Alternatively use Windows' offline files feature when you are away from the fileserver.