NAS enclosure vs linux server for backup storage

NAS Option: Pros - Easy to hookup, 10 times faster than gigibit ethernet, no new server needed. Cons - Physical server presents a single point of failure.

Debian Option: Pros - You get a new server, you can double up the RAID controllers, run RAID 10, you can install as many NICs as you need. Cons - Single point of failure.

My Recommended Option: (Since you have some $$$ to spend)

Keep your existing backup server, procure the NAS devices for storage, add a NIC for every subnet in your infrastructure, and connect your backup server to each network segment. This will optimize your incoming data flow when your are preforming backups and not tax your network. (Not needed if you have a dedicated backup network segment)

I would also build a duplicate backup server that can serve as a hot standby for any future failures. If your production server fails you simply move the NAS devices to the "backup" server.

Also I would create 2 backup jobs per server and have them run "every other" period so you get backup data on both of your NAS devices. Takes up extra space yes, but you can manage this fairly easily.

A final option would to do a cost comparison and see what your cost difference would be for one of the many available cloud backup solutions with your data only being under 15TB.