v4. More efficient. RAM will be fine. I believe the ‘stock’ speed for DDR4 is 2133.

That’s something that I’ve never tried before.

I’m more so worried about performance loss having a SATA SSD mixed with an NVMe in parity. You’re talking a 500MB/s SATA SSD vs a 3000+ MB/s NVMe.

I don’t know if I would combine NVMe and SATA SSD’s.
You need a boot drive as well, so one will be the boot drive, then 2 for the storage array (assuming you don’t boot from USB or something)

I personally would boot from the SATA SSD, then mirror the storage disks if redundancy was needed, or stripe if you don’t care for redundancy.

If you don’t mind building your own rackmount server, I can wholeheartedly recommend Sliger cases. They have just about everything 3U/4U and lots of different depths.