However, I did look for 2.5" drives (the server comes with a 2.5" backplane) and there don't seem to be many "consumer" drives in this format that are both high capacity (>2 TB) and non SMR (I'll be running ZFS). I'm also not looking to spend very much, so SSDs are out.
[0] Seagate specs for 2.5" Barracuda: https://www.seagate.com/www-content/datasheets/pdfs/barracud...
The startup current is 1.2 A under 5V. So if you're running four of them, you'll need 24 W just for the drives. Not sure if the "cheap" enclosures are able to stagger drive startup.
My Gigabyte motherboard cannot, so it would require a PSU large enough to drive both the board (CPU + RAM + etc) and the drives for a little while. So with four drives, you'd be looking at 50 W to be sure it fits.