Consider how the NAS will be used. Is it just file storage, or will you want to stream from it?
If just file storage, you can use lighter hardware.
I’m running a 5 year old Dell Small Form Factor desktop as my NAS/media server. It’s power draw is under 12 watts unless I’m converting files. There’s room for 3 data drives (boot drive is M2). It has no problem streaming, unlike my consumer NAS. And it cost way less.
Also have a backup plan.