I would use Intel as a motherboard, given that asrock is a good option lately out of seeing what some have accomplished, but still intel has the track of most of the builds bieng the lowest in power consumption.
The 10W seems reachable, but you gotta remember that consumption is not steady, it will jump to 15W and back down on a windows evenoirment, now if you use another OS, you should check first if it will allow the downclocking, as this is only reachable when the CPU is clock at 800mhz.
Now 15W with 2 mechanical.... i doubt it, unless they are slept, the consumption of those hdds not even in use will be 5W+, so expect in the 20s is my guess with 2, in use they will probably jump more.
Personally i like more to have my download station separated from my storage server, at the beggining, i though the same, leave the storage server, so i bought an Acer 342 atom based, but upon loading more drives on it (only 4), the idle went up to 37W, and i quickly filled the 4x 2tb, so i decided to move create a server for pure storage, i started with 8x 2tb and 6x 1.5tb, with the xeon on idle i get around 125W (no hdds slept), but i didnt want to leave a 125W server on for downloading, so i created the download station, that usually idles around 10W, but thats my story, now im moving toward 4tb my server and moving into WHS2011, i just been too busy lately to put some time in the server.
Either way, if you are able to sleep the hdds i think you will be able to get low power consumptions, else adding 3.5 hdds increases the consumption.
Good luck with the build.
GameMi >> MSI Z87-GD65 | Intel Core i7-4790K | Thermalright Silver Arrow IBE + 3x Noctua NF-A15 PWM @500rpms | Crucial Ballistix Sport 32GB DDR3 1600 | nVidia GTX780 + ARCTIC Accelero Xtreme IV | Samsung SA850 27'' 2560x1440 | Samsung 840pro 512GB | Hitachi 7K1000 1TB | Fractal Design Define R4 + 4x Noctua NF-A14 PWM @450rpm | SeaSonic SS-860XP2
Other builds ---> ServeMi | CamMi | MiniMi | HTPCMi