I’d probably do 4 x 10 disk RAID Z2 vdevs. Probably just under 600TB useable with 20TB disks.
This all sounds like pipedream stuff though, to have that much storage requirements but not really have any idea of RAID, parity etc. and you’re somehow also backing this up somewhere too.
No QuickSync on Xeons. Use the Optiplex.
Maybe find a cheap E5-1680 v3, and replace the 4gb sticks with 8gb.
That’s ancient, so no it’s not going to be able to handle that.
I’d definitely be running Proxmox on all those Dell’s and virtualise all those apps. Makes life easier backing things up and moving to another node if one fails.
The Dell for sure.
You’d likely be limited by CPU before you can use 128GB of memory on the Lenovo.
2 vdevs with 10 x 10TB drives in RAID-Z2 would give you around 150TB useable.
So you could do that with one vdev and 10 x 20TB disks, but you wouldn’t have as good performance and 20TB disks take a long time to resilver if you have to replace one.