The easiest way might be getting an ITX board with N100/150 from CWWK or Topton. They usually come with 4-6 SATA ports and the CPU soldered on. Then you just buy your favorite case and plug in your drives. Excluding the cost of the drives you might end up spending about $250, depending on your case and PSU selection. The problem with using SBCs or mini PCs that way is that you need an enclosure to hold, power, and cool down your disks, and even that is assuming that you can actually connect them via a SATA/SAS connection which may not be easily available from within your case.

Check out Hardware Haven on Youtube, he tests a bunch of different small form factor NAS computers and similar. I find the whole "raspi ecosystem" a bit overrated, for the most part it's Linux you're working with regardless of hardware.

If you go with x86 you've got a bunch of people doing it under the "homelab" banner, so you'll hardly be alone.