Consider humans when designing any backup scheme. People will delete entire filesystems by accident, and any raid solution will immediately replicate the deletion. They'll have deleted a file a month ago and want it back. They'll have overwritten a file with bad data and want the old file back from last week. This human may be you.
Hardware failure is real, but it's probably the least of your worries if you're discussing backups and not availability.
All good points Matthew; I imagine that a Time Machine type of setup would be ideal, however I don't know whether that works over the network.
After some googling, it appears as though
these guys reckon it is possible to take the disk(s) out of the NAS and read data directly from them.