Is OS X Server the best way to go?

[brainwave89]

My wife has a four person law office. All of the workers needs to share tasks, contacts, documents and calendars. They also need to access their documents outside the office.

One of the four will be getting a new iMac shortly. The other three are currently using PC's.

Does it make sense for all of the users to use Mac's and get a Mac Mini with OS X Server? I presume using the server they can satisfy all of their needs.

I want to make it easy and effecient for them to do their work. I also want to be ahead of the curve if and when more people are hired. I'm also concerned about maintaining a high level of security with their information and day to day operations of their computers.

Any feedback will be appreciated.