Hi. I've set up multiple Mac minis with OS X Server in the past years. All of em have used it for contacts, calendar, file sharing, VPN, messages and a few other services. I have to say that OS X Server is, by far, a rock solid stable and amazingly easiest to set up server product out there. Ive set up OS X server in the following:
1) A small law office with 4 employees
2) A computer service company with 8 employees
3) Another small law office with 4 employees and 5 outside employees who access the very same server remotely
4) A health and well being company with 23 employees and 2 outside employees who access the server.
They all have variations of Macs, iOS devices, PCs and all of em are able to access and use the server efficiently. I would recommend getting an Airport express router because setting up the ports for the server will be just a 2 clicks away rather than having to look for port numbers and manually forwarding them (its just a matter of simplicity). But I highly recommend you to get a Mac mini with OS X server, it is an amazing machine.
A Mac server is easy to setup and cheap, a Mac mini server also is very quiet, small, and uses very little power. From these points of view it is an ideal small office server.
However you say 3 out of 4 of your users are PC users presumably Windows users. While the Mac server can act as a server for email, file serving and even VPN for both Mac and Windows, it by itself is less suitable for use as a contacts/calendar server as it is not compatible with Outlook for these functions. There are alternatives to Outlook for Windows e.g. Thunderbird, SunBird, etc. which can do some of these functions. However another option would be to use Kerio Connect as the email/calendar/contacts server as this is compatible with both Macs and Outlook for Windows.
Note: I have discovered what I consider a bug in Mountain Lion Server in that even if you don't use it as a web-server but do use it as a VPN server, the VPN server does use the http port. This would block the ability for Kerio to be able to provide a webmail function. I have reported this to Apple.
I am wanting to setup my mac mini with OS X server as you described above. We are a small company with 15 employees. Mostly macs, but a few pc's. I want to be able to have file share mainly. Possilby in the future have e-mail on our server.
Are you available for-hire? I am not up to speed when it comes to setting up the server.
I like the idea on the Airport Express. I will be upgrading to one.
My contact info is:
firstname.lastname@example.org if you are interested or know of someone who is capable of helping me with my server setup.