What you linked is going to be very close, enough you won't do much better or worse unless you went with more power hungry nVidia chipsets. Since it's Asus that also increases the odds of having over(under-)clocker options which will make the most difference, since you don't need even 1/3rd the performance of a modern system for a NAS and by underclocking you easily save the most power of any choice you would make.
For comparison purposes, an entire standalone NAS may consume under 10W total till you put a HDD in, easily less than 1/5th the power consumption. Typical affordable standalone NAS is a lot slower over GbE though.