RAM is accounted for in GP's description.
Regarding the boot drive, you can either boot from the data disks (say via a common ZFS pool or a common raid + lvm) or if you're happy with a SD card for the PI, I suppose a small USB drive would do the trick.
Regarding the PSU, but you'd need a beefy adapter anyway for running 4 drives, which isn't cheap, and there are cheap pico-atx PSUs available. I bought a compact case + PSU for my board for 50 euros around two years ago [1] that's still going strong (although I'm only running a single SSD in it), so the PSU alone should be less than that.
There's of course the power efficiency question and I presume this setup would be somewhat more power hungry than an RPI. The Intel specs for the J3455 give it at 10W TDP. I have no idea how much RAM consumes, but it's likely a few more watts.
[0] https://www.gigabyte.com/Motherboard/GA-J3455N-D3H-rev-10#ov
[1] https://www.inter-tech.de/en/products/case/mini-itx-nuc/itx-...