iMac is ok for all 2k work, for heavy 4k full production and colour grading work maybe a little too limited (GPU max 4 GB, 8 CPU cores).
Starting with your 5 TB documentary data a 12 TB Raid with all it's overhead doesn't give you a lot of future growth, watch out...
I'm not a big fan of running a Raid with consumer disks, look if the offered discs are raid enabled, raid certified and/or prepared for 7x24 operation.
I only can recommend to operate in Raid 6 mode, i.e. in the case of a disc failure recovery the dataset is still protected. The link below discusses also the topic Pegasus vs Areca.
PS: I run a late 2012 iMac 32 GB main memory, 2 GB GPU, 24 TB Raid6 storage from Areca
Thanks for your experienced and thoughful reponse.
I was thinking that a RAID 50 might be good because it would give me 8TB instead of 6TB. Have you any thoughts on that? Seems like it would give me more room, and redundancy and speed.
The Areca may be out of my budget. It looks like a good system. Is there something that I'm missing? Larger drives in a 6 bay system like that might be better. Is there anything that would give me larger space but still stay around $1300 as a ceiling?
the financial boundary is (as always) ruling the decision... Invest in any case in a real physical backup on top of your RAID-array to protect your data in case of a raid crash.