i think the biggest issues is removing heat from your case. without a fan the heat could build up and slowly cause the fanless heatsinks to increase to an intolerable heat. i think a principle that would be important to keep in mind is, heat rises. i would look for something that vents that heat naturally. i think choice of case would be as important as choice of pc parts.
I have a lot of experience with what you folks are saying. Rockdpm had a good thought, to watercool it. JSC mentioned you had to minimize all heat. otester was going to a lower end processor, which is good. Everything suggested here is good. Neon Neophyte was very correct, the case is critical. We finally have a case that is completely fanless with a full Intel Haswell i7 vpro technology processor. The case is completely sealed and looks like a giant 7" x 8" heatsink! It runs great and we have spent tens of thousands of dollars developing it. My VP is VERY proud.