The microsoft stack does work quite well, but usually for 50+ staff. For less than 50 the cost of administration becomes a disproportional overhead.

I take vendor lock-in with a grain of salt... if the solution works well, the lock-in (almost) doesn't matter. When it comes to mission critical business systems, most companies have vendor lock-in, whether it's with Microsoft, Google or some other company.

I think most startups stick with Google Apps. For network filesharing have you tried Dropbox? If large files are an issue - you will need to setup a fileserver in your office something as simple as a samba server could do the trick.

You could also trying to get an invite for AeroFS.