I'm searching for a secure and easy way for business partners to collaboratively edit and exchange documents, share calendars, create schedules, and assign tasks. I speculate that the ideal collaboration environment or work-flow would actually involve several technologies and services. My co-workers and I have tried a variety of things from Google Apps to Wiki's, but nothing feels very fluid or complete.

I suppose defining what we need and our constraints is probably in order:

1. collaboratively edit basic text documents and spreadsheets
2. exchange documents like flow-charts, graphs, and files generated by our other desktop applications, but not source code
3. assign tasks to each other and ourselves and track the history of those tasks
4. easily see when relevant documents have been modified since last viewing and ability to easily push notifications to relevant workers (a clean front page that shows updates would probably suffice)
5. provide limited access to contract workers and guests users
6. if a remote user system is compromised (keystroke logger or other spyware) we don't want the criminal to be able to gain access to all business documents (processes, trade-secrets, customer lists, etc.) simply because they gained access to a single Google account (or whatever web service)
7. Cannot be a difficult to administer VPN infrastructure
8. Cannot cost more than $100 per month (yeah, money is tight)
9. Needs to support up to 25 users
10. We can host our own web applications, but it must be low maintenance solution