15min with a sharpie, a drill and tap says any 2U case fits as many ITX systems as you can physically fit in the space. Then probably 2-3 hours working out the wiring so the redundant psu's power them all.

Why bother with multiple motherboards when you can just have 1 and many VM's?

Some Dell servers require HDD/SSDs with Dell firmware locking you to their ecosystem. No idea about workarounds but I assume they exist when the factory warranty doesn't matter anymore.

Dell should tell you in the specs if you input the service tag to their service tag finder.