I have a homelab with a bunch of old HP Gen 8 Microservers. They hold 4x 3.5" hdds and also an ssd (internally, replacing the optical slot):

https://www.ebay.com/itm/156749631079

These are reasonably low power, and can take up to 16GB of ECC ram which is fine for small local NAS applications. The cpu is socketed, so I've upgraded most of mine to 4 core / 8 thread Xeons. From rough memory of the last time I measured the power usage at idle, it was around 12w with the drives auto-spun down.

They also have a PCIe slot in the back, though it's older gen, but you'll be able to put a 10GbE card in it if that's your thing.

Software wise, TrueNAS works pretty well. Proxmox works "ok" too, but this isn't a good platform for virtualisation due to the maximum of 16GB ram.