1. Google Apps for Business - No onsite hardware - Calendars with sharing - Webmail / imap access - Can be a central point of administering users, if the other apps integrate to Google Apps (e.g. the Apps from Apps Marketplace) - Supports Windows, OS X and Linux - All machines are fully usable outside the office

Now you are almost there. Forget that MS stack. It's costly, vendor-lockin, and feels like a thing from a past.

2. Central Management of Users with Single Sign-on capabilities. - Look at http://www.okta.com/ and http://www.symplified.com/

3. Network File Sharing - Google Docs (and there is a change that Google finally introduces Google Drive / Google Files)

If that is not enough, then look at:

- Egnyte: http://www.egnyte.com/ (Hybrid solution, Dropbox-like sync-client, Client Backup, Local office sync server / NAS, FTP (includes secure connection) access, WebDAV access with Drive mapping capability, browser access etc.) - Box: http://box.com/ - Dropbox for Teams: http://www.dropbox.com/teams

If you are interested in backups, look at: - Backblaze: http://www.backblaze.com/

Remember that with Google Apps, you already get very nice features, like: - Single sign-on on every Google's service (for example: Google+, Google Voice, and you get services like Postini, Mobile synchronization with support for Active Sync, etc.)

If you are going to MS stack, you are going to tie your hands. It's easy to implement MS stack, but it's damn hard to exit their system.