While I agree that scattering data over systems in a workgroup is a horrible I idea, the suggest to implement a linux server is not appropriate - UNLESS you have experience working with linux. The supposed savings over Windows licensing fees will be RAPIDLY eaten up the first time you have to get support because something stops working appropriately. Free of license fee DOES NOT mean free of cost.
If you plan on hosting e-mail then you should absolutely strongly consider SBS. If not, then you shouldn't. SBS is an integrated bundle of Exchange and Windows server that offers additional Remote Access and Management Wizards for ease of use. The price of it is significantly discounted from the regular retail products. But if you don't need Exchange, then you're better off not using it. I would suggest, if you are not looking for Email, you consider Windows Foundation server - this product is designed for networks of 15 users or less and comes pre-installed on major brand servers (Dell, HP, and the like). The license fee is a relatively low $300 or so and there are no CAL licenses needed.
The benefits of using a server is a centralized login for all workstations, even the Mac, when it comes to accessing network resources. In addition, a windows server offers (when configured) Shadow Copy which is a periodic snapshot of all files on the disk, providing a "version history" of sorts for recovery - this should NOT be used as the only backup, but can be a handy backup for files recently deleted or accidentally changed.
A NAS device isn't a bad idea for a small network, but if you go that route, you should MAKE SURE you get one that supports RAID and ideally a similar snapshot functionality. Such a device can be almost as expensive as a cheap server.
