What about a genuine new server board?
(Now the price is interesting… It used to be less than half of that—and was 55€ with a bundled cooler the last time I posted this link. We have an anti-Black Friday operation here. Feel free to lowball.)
Now you’ve got some build issues. Proxmox is a dangerous hypervisor for TrueNAS. You need to passthrough the drive controller—and that’s still not enough. A HBA, a NIC, and then a dGPU and some NVMe drives is more than what a consumer-grade CPU can properly handle.
If you want to do transcoding, consider going for Intel and use the iGPU. (Otherwise an Arc 310 is small, possibly even single-slot or half-height, but still take some CPU lanes.)
New HDDs are generally preferred: After all, this is about reliable storage so don’t cheap out on the very foundation.
Model: Anything that is NOT SMR (Seagate Ironwolf (Pro) or Exos; Toshiba N300 or MG; WD Red Plus/Pro, WD Gold or the corresponding HGST Ultrastar).
correct