How much budget are we actually talking about here? Any possibility of getting two ZFS devices, using snapshots, and mirroring the two boxes (and their snapshots)? That gives you redundancy and failover as well as solving the backup problem. Probably looking at around £60k (at a guess) for both devices though.
If you're on a budget, have you looked at building one of these: Petabytes on a budget: How to build cheap cloud storage | Backblaze Blog
1. Good advice above.
2. Think about the tradeoffs of not having that data backed up
3. If the data's not critical then just use RAID, if it's critical, do you need it in case of a site failure, from the post it seems not as the data was in cupboards anyway
4. If you have SAN, you can use RAID with SAN replication if you do have another plex.
Box of 2Tb HDs and a USB caddy - sorted!:cool: