I'm in process with one as well. Building the box its self was easy enough, but ironing out the electronic specifics has a few pitfalls.
Going the Cooler Guys route seems to be the easiest plug and play solution, but if you get the controller, power supply, fan(s) and extension cables from them, it adds up to a fair amount of coin. I was tempted to do this, but then I thought, If I'm going to put more than $100 into a foam box that I have to keep shovelling ice into, I might as well spend just a little bit more on a 2nd hand chest freezer and JC.
Anyone that has an old scrap PC lying around could just strip the cooling fans and power supply for free...that's what I did. An old unused cell phone type AC/DC wall warts work just as well, but keep in mind that most of those don't put out a full 12 volts, so the fan will run proportionally slower with a lower voltage. That's not necessarily an issue for this application though.
Finding a reasonably priced thermostat that meets all the necessary design criteria is the tricky one IMO. Most of the ones that were used when this design was first popularized have long since been dicontinued. You want one that has battery backup, and the ability/option of powering itself from those batteries, not just as a memory back-up.
I just picked up a Honeywell RTH111B for $20, but upon dismantling it, I was dissapointed to learn that extending the thermister is a no go. This newer generation of stat utlizes miniaturized sensing components that cannot be extended by removal and wire extension. It will still work fine if I mount it inside the ferm chamber, but I won't be able to monitor it without opening the box up and losing all the cool air. The other drawback is that I won't be able to monitor the temp of the beer, per se, just the air around the beer. I can work around this with a turkey fryer thermometer pushed through the top of the box with the tip submerged in water within a yeast vial, just like you'd do on a keezer or kegerator. That should better approximate the slower change of temp through a liquid and keep the fan from cycling more often than necessary. I can then monitor the actual wort temp vs. the thermostat sensing, and dial that down accordingly.
HTH