I'm a bit of a skin flint so wouldn't go Windows

If I understand the licensing rules properly, you're looking at GBP8150 for Client Access Licenses ($9950 if purchased in the US) + the operating system cost, + the hardware.If you want something other than a non-expandable off the shelf appliance it may be worth you looking at something like FreeNAS or Nasforfree instead of a Microsoft solution. You can read a bit about them on the Distrowatch site. Using one of these you'll still get the huge advantage that a NAS box provides - no malware breeding on the device - but can choose your own hardware and add more disks as you need. Oh, and as the names suggest, there isn't a cost for the OS.