Best shot at a low-power, fanless NAS?

[nanoserver]

Starting from a Fractal Design Define R5 case, I'd like to know my options for building a low-power system almost exclusively used as a NAS, with the following guidelines:

- total idle power draw of a few tens of watts or less

- passive cooling would be awesome; is it possible at all?

- using more-or-less regular components as opposed to, e.g., specialized industrial motherboards

 

Should I be aiming for i3 CPUs? Older i3 CPUs versus newer i3 CPUs?

What should I look for in a motherboard?

Should I expect to tweak lots of BIOS and/or Unraid settings?

Do modern PSU with different "color grades" (gold, etc.) really have different efficiencies when handling with low power draw? Should I look into external PSU options?

 

Thanks for any pointers!