I recommend going with an N100 based system if you want low power usage. I doubt old workstations that support multiple drives idle near 10W, the mini PC variants might, but they would require a DAS for the HDD’s.
I have personally chosen an ASUS N100I-D D4 motherboard with a m.2 to 6x SATA card and it has been working fairly well.
I want to look into quadlets