Welcome to the forum!
You pretty much can't go wrong with Supermicro and a Xeon :) Not only are Supermicro motherboards the gold standard on here, but Supermicro chassis hold a special place in certain member's hearts. Just be sure to read the guides that are available, especially cyberjock's presentation he put together. A handful of the pros on here have spent a lot of time putting together not only hardware recommendations, but just general best practice for configuration and use of FreeNAS. Humbly follow their advice and you'll be set :)
As for the RAM, there's been a lot of buzz about Kingston RAM. They used to be the go-to RAM for FreeNAS builders, but apparently they've been caught doing some shifty things with quality of their SSD's and apparently their RAM is off the suggested list. People seem to be looking for Samsung or Hynix ECC lately.
That said, I did my build back in May and bought Kingston RAM (it seems like that's about the time the concerns started coming up, and I didn't find out about it until later), and it seems to be doing just fine. I wouldn't risk it though. Kingston is the ECC RAM that's easy to find, but look a bit harder to find a set that's going to be more reliably good.
Also, one important consideration is to get an UPS. With all that data being constantly backed-up, it seems like it would be extremely undesirable to lose it all from a power outage. That won't always happen, but it can. It just seems silly to spend all that money to build the server but not add the extra layer of protection you get from having an UPS.
You also know that having 21TB of disk space in RAID-Z2 won't give you 20TB of storage, right? Just checking :)