A lot depends on the speed you need. One of the simplest systems is JBOD USB 3. An enclosure like this. Get an eSATA card for your Mac Pro and then if you upgrade use the USB 3.
https://www.amazon.com/Mediasonic-HF2-SU3S2-ProBox-Drive-Enclosure/dp/B003X26VV4/ref=sr_1_11?ie=UTF8&qid=1391699242&sr=8-11&keywords=jbod+usb+3+sata
USB 3 is plenty fast for individual drives.
Before we had centralized storage we used bare drives. We never deleted anything from the drives just added new drives when they became full. We basically have the same system now, but with partitioned drives on a NAS system backed up to individual drives. We are now up to drive 60. The advantages of this are simplicity, unlimited expansion and instant recovery if anything goes wrong.
So each drive has two mirrors, one stored off site. We run nightly backups on the active drives so they stay the same.
If you want a thunderbolt RAID 5 enclosure then I would partition logical drives, say start at Drive01 for 4 TB. Then have two 4 TB bare drives for backup. Run a synchronization each night to one of those drives. Store the other one offsite. Then swap the two periodically. Chronosync is a good program to handle synchronization. Once Drive01 fills up, make sure both drives are synchronized, then delete the partition and add a new partition, say Drive02. Continue the process. So active projects are always on the RAID. Older projects are on two backup drives that you can mount with a USB 3 enclosure. One is always stored offsite.
Hope this helps.