Five or More Workstation

NAS (Network-attached storage) technology continues to improve and because the Linux operating system is 'open-source' and free to distribute, we wouldn't be surprised if they begin to take over in many server environments; but they're not there yet.

On a NAS or Professional Version of Windows three would probably an upper limit before we recommended a proper Windows server.

Windows Server 2012 comes in 4 basic versions: 

Foundation, Essentials, Standard & Datacenter

http://www.dell.com/us/business/p/poweredge-t320/fs

(note the prices show without a selected Windows Version)

We recommend 'Standard'; although Essentials may fit now, it is missing components that may be useful later.

You'll want a network expert to help with the purchase and installation.