Or look at the
Intel DQ77KB and pair it with a low-power E3 CPU (like the
E3-1265L v2). This will give you <20watt idle, vPRO AMT for remote access, but only a single x4 PCIe slot.
Is the idle power from experience? If so, what OS?
How much would a system with Supermico X9SCL+-F and a E3-1265L v2 8GB ram and SSD use at idle? Please look away from the Unraid HDD's, as these will spindown when not in use.
If you look at the main site reviews of the E3's, Patrick gives idle and load power draw. If you don't expect the CPU to be loaded often, most storage servers in my experience, I estimate any of the E3V2's will idle <40W. My desktop (E3-1245V2, 16GB, HD6950, Intel 320 SSD, ConnectX2) idles <50W when I can get the HD6950 to down clock. The config Piglover suggested is pretty amazing for idle power, especially if using a non-windows OS.
Personally I would go with Supermicro X9, an E3-1240V2 and the smallest gold/platinum rated PSU you can find, like
this 200W Gold Supermicro or the Supermicro PWS-341P-1H 340W Plat. 200W is cutting it really close, 150W conservative max for CPU, ram, and SSD and ~60W for four HDDs.
You need to be able to route to the IPMI IP to get to it. If you are virtualizing your router and the box dies, you are out of luck. You may look at doing a dedicated router/firewall appliance that can be a VPN end point.