Mac Backups - 5xMac's with a synchronized catalog?
I would look at getting a NAS (Network Attached Storage) box which appears to be basically a disk that can be accessed from anywhere on the network and is added to the network like any other machine. I would suggest using Ethernet to the router rather than WiFi.  (I just got a very simple 2TB NAS for £80 so can be inexpensive, although given this is a work setup I would expect to look for a more complete set up costing more, remember to work out the cost of losing a design before saying any solution costs too much)

In the simplest form a share can be mounted to each Mac where it will look like another disk in the finder. Then users can copy from their machine to the NAS.You would also have to set up another process to backup the designs from the NAS. Most NASes come with software that sets up automatic backups from Macs to the NAS. The other benefit of having the data on a NAS and not on DVDs is all the designs are quickly available. Your X11 catalog should also be able to work off this.

More complex NASs will include things like 


RAID which means you don't lose data when one disk fails.
allow you to add more disks to increase capacity
Do unattended backups to cloud or other off site storage
Could burn DVDs itself