I spent quite a bit of time thinking about hardware for a file server. My use is for a couple of layers of backup for the active machines, with one server on 24/7 and another powered on intermittently to back up the 24/7 machine.
In fact, what I was looking for was ZFS for the data integrity issues. That led me to OpenSolaris, which is where I would have stayed had not Oracle employed its little bit of social engineering to run people away from the free stuff. I took the hint and left. Still looking for ZFS, I came to FreeBSD, and then FreeNAS, which does what I wanted anyway.
Since I was looking for data integrity, I decided that ECC RAM was a critical issue. ASUS offers several motherboards which let you use ECC RAM *and* enable that to actually do ECC, which are different things. Some motherboards which say "ECC" are actually only ECC-tolerant. The reason ECC is important is that a good disk subsystem will faithfully store any incorrect data you feed it from a motherboard RAM error. Soft errors in RAM are not uncommon.
This constrained my choice of motherboards and processors. Intel has printed into hardware the inability to use ECC unless you use their high end processors. AMD on the other hand makes ECC available for all but the lowest mobile processors. I dug hard into the itx fanless motherboards and found none of them that supported ECC; otherwise, I'd have gone for the lowest possible power solution there.
But ECC put a minimum level on the choices. I went for a MATX with ECC and integrated video; no reason to spend the power or money on a video card. It had multimedia stuff which was immaterial. It also had six SATA ports and one IDE port.
The IDE port let me put an IDE to compact flash adapter in there for a boot disk. Cheaper and lower power than a SSD. The six SATA ports let me put a useful amount of disk storage on line without additional adapters.
The power budget let me run the motherboard and all the disks for a measured 76W of power (as measured with a power meter at the AC power socket) at idle, never peaking to over 100W, even at power up. I plan to worry about disk spin down at some point.
As it happens, the power supply for a system like this gets problematical. Low power ATX power supplies are not particularly efficient. You can actually spend as much waste power with a lower power, but lower efficiency power supply than a higher power, but higher efficiency power supply. I wound up with high efficiency Corsair 400W supply. It's losses are in the 76W.
I know that I could run more disks. The peak power comes from disk activity. Each disk has a power peak rating for startup and for runing. The startup peak is the real issue that limits the power supply. You have to have enough power supply rating to spin up all the disks simultaneously unless you have a staggered spin up capability. I know how to do staggered spin, but it's not a standard feature of most motherboards, disks, or I suspect FreeNAS/FreeBSD. Since I had to buy a bigger power supply anyway, I chose not to worry about that point.