Thank you all for your suggestions. The SuperMicro X10SBA seems to have a very good feature set for the price, however it looks that it only supports a 32 bit UEFI bootloader, see: https://forums.servethehome.com/index.php?threads/how-about-some-bay-trail.2828/#post28536
This causes problems when booting a lot of Linux distro's. I am surprised it works out of the box with FreeBSD 8.3, looks like FreeBSD supports the 32-bit UEFI mode. However, would this be the same for future versions? Perhaps SuperMicro may release a BIOS update which may fix this issue, but I have not seen any post anywhere confirming this.
Another option I am considering is an Asus H87I-PLUS ITX or Asus Q87T/CSM Thin ITX motherboard with an i3-4130T. Reports show it should run around 20 watts idle with a PicoPSU, probably lower for the Thin ITX. With some further undervolting and underclocking, this may even be further reduced. On the plus side, this CPU would be more future proof given its higher performance and it also supports AES-NI.