My thoughts about the OS:
FreeNAS is a great solution if you (a computer professional) will do the management and keep a watchful eye over the machine. Should the server be "managed" from the typical Windows power user, I would certainly choose Windows server instead (the low-cost version, eventually). Windows server has many safeguards that prevents the would-be administrator from shooting on its foots; basic issues such as the user directory and security are working satisfactorily with the defaults. The Windows 8 built-in file history and offline folders are simple to use and they make a good enough backup system for most people. There are many auxiliary features that could be useful in a small office.
About the hardware:
I have bought a dozen microservers for the company where I work and I use one at home myself. They are a really good cheap server, but aren't such a great enterprise server. I would rather avoid them, if the budget is at least decent. Microserver's CPU is weak and the hardware quality is... well, not the best in town.
On small offices I preferentially use machines with dual power supply. People can exchange a faulty UPS without service disruption, and a clueless user will not disconnect both power cords when it needs to connect the phone charger to the wall outlet ,-) If this option is not available, I lately specify the Dell T20 (the xeon model, with ECC RAM).