You might want to change the last page. DVD is a compressed format
If you decompress it 3.5TB will only store about 30 hours.
So, can you fit a 6th drive using the eSATA and make a 6 disk array?
And have you tried using the GbE bonding? At home my GbE can pull 80MB/s over the network. I suppose an areca card with Xscale at 500Mhz (or was it 300?) could do RAID5 at 200+MB/s the Celeron M should be able to do better. Dual GbE should be able to pull somewhere around 140MB/s?
600 pounds for the NAS is a bit too expensive IMO. if they fit an areca card into the NAS (and let the Celeron M handle the network/os/usb) then it would be bargain