And 4 digit names I think are the and variants. Also , an r740 usually has 8, but can have 16 drives, in the 2.5" variety. plus boot drives. An r740xd can have 24 2.5 or 12 3.5 drives.

Then you have storage variants, think end xc. An r340 likely has 1 cpu, few drive slots. An r640 is 1u. An r740 is 2u An r930 is 2u, 4cpu (did the do 940, 950 variants?)

Icydock makes a dual hotswap m.2 card. They pop out the rear. Don’t think it does raid. Maybe is close enough

Can’t help with these 3 specifically. I ordered som wd gokd drives. Which are the ones intended for datacenters. Price was lower. And I think it was 10^16 not 10^15. That may not sound like alot. But doing the math, number of bits on a raid6 of 8 14tb drives, basically going to hit 10^15. And they supposedly have firmware to handle vibration. Maybe worth looking into.

As for these 3, I would think they would all be fine choices.