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 have 2x 2658v4 in my Dell R730. Idle with no spinning rust, an old Quadro, and a couple NICs is ~120w.

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.

Possibility your RAID card was flashed to IT mode/with LSI firmware?

The RAID utility won't work if it's not running Dell firmware.

You can adapt the front DVD drive to a 2.5” SSD or you can use an NVMe drive WITH a USB drive with a bootloader on it (clover or rEFInd)

R710-30 do not have options to boot directly from NVMe, but you can use workarounds (like above)

The 600 series are all 1U, they will be obnoxious if the fans are spun up high.

That said, if you’re not going to be hitting the CPUs very hard, you can use iDRAC/ipmitool to control the fans and bring them down to 10-15%. At that level, they will be an audible hum, not super loud.

I would recommend a R700 series since it’s 2U, more heatsink, larger fans, you can get away with turning the fans down more.

If you’ve got the serial cable, use PuTTY (or your preferred client) and see what the output says.

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.