Speaking as someone that does not manage a server (where is MrHoffman when you need him?), but having worked with Macs for years, and I play at being a Unix file system developer by day...
Everything rccharles said is good stuff.
File storage will be external and routed to a NAS with RAID 1 or 5. Recommendations as to best product?
For NAS, Drobo is painless adaptive RAID technology. You plug in the drives, and if figures out based on number of disks, and what sizes they are there which RAID technology to use. They offer single and dual RAID 5 parity drives so a single disk failure never leaves you without protection. Failed disks can be hot swapped and the Drobo will re-silver the replacement drive. You can migrate to large drives one at a time, by pulling a smaller drive, popping in a large drive, and wait for it to re-silver, then wash/rinse/repeate until you have migrated all the data to larger drives. They have 4 drive, 5 drive, and 8 drive enclosures. Some units are USB3 connected, and they have some that are network connected (I think they even have an iSCSI network interface along with the iSCSI drivers available; never used it myself)
For home users that need lots of protected storage, they are great, as they are relatively easy to manage. I will not try to speak to commercial use, as I have no experience managing a real server.
And, yes, I have a few 4 bay, USB attached Drobos.
Synology is a well respected multi-drive NAS that works with the Mac. Synology has the ability to backup to another synology unit. That might be useful for your backups.
I have never used a Synology, but a Mac oriented podcast I listen to, loves them, which is where I have received all my information.
What about backup designs?
If they are truly NAS connected over Ethernet, then backup might be best of the NAS can do its own backup to another matching device.
If you are going to go through the Mac, then TimeMachine for Mac formatted drives.
SuperDuper (as mentioned by rccharles) is again good for Mac formatted drives.
As is Carbon Copy Cloner (very well respected Mac backup utility based on rsync, which the CCC author contributes to so that it includes Mac file system feature support). CCC has a little more flexibility for selecting subsets of a directory tree to backup. It can also do a backup over a network rsync via ssh connection to another Mac.
However, if you are going to use a NAS, and not direct attached (USB, Thunderbolt) or iSCSI storage, then a NAS often times formats the storage their own way, and it might be better looking at what the NAS offers for backups (as I mentioned for Synology).
Again, maybe asking in the above mentioned macOS Server forum would give you more useful replies from people that actually manage an macOS Server setup for real.
