It's not new or particularly fast, but the Supermicro X9SPV might be found for a reasonable price, I'm pretty sure it will idle under 15W, though I've never checked mine. You just get a single PCIe slot, but it is at least x16 and it does support bifurcation, so with some sort of riser/adapter you could get two x8 cards working (just select them carefully to ensure ASPM will work to keep idle power low.)
There are several Atoms that support ECC, but quite a few of them are possibly affected by the AVR54 bug so it's a little risky buying them, plus they're generally pretty limited on IO and expansion, and if they include IPMI then that uses about half of your desired idle power on its own, doesn't leave much for the rest of the components.