> It also turns out that 32 HDDs only consume 200w, so the 750w PSU we picked is more than sufficient.

Yep. Stopped reading right there. HDDs use ~15 watts when they boot up. I experienced this and I never allocate less than 20 watts / hard disk.

Staggered spin-up is a thing. That motherboard doesn't have it most likely, but they also can't plug 32 HDDs into it either. They'd need an HBA card of some kind, which is more likely to have staggered spin-up.