I would certanly look at FreeNAS as an OS for such a device. I have ran FreeNAS on a compat flash card and used it to setup software Raid arrays that are accessable as either CIFS shares or NFS or Rsync hosts.
There is no right answer. here is an interesting bit of kit from <a href=”http://www.via.com.tw/en/products/embedded/nsd7800/”>VIA </a> it retails for about 500GBP and you can just pop your own linux distro on.
Hope this helps