Why the need to get the temperature below 60 degrees? Is it so you can lager in it? My understanding is that for ales, you don't need to go below 60. I had thought about building out something like this as well, One of the concerns would be for even heat and cooling distribution inside the chamber.If you have the air too close to one of the buckets/carboys, that batch will have a more drastic heating or cooling change compared to the other batches. I was thinking that the cooling unit or heater should be either on top of or below the buckets, to allow the chamber to heat or cool uniformly. I'm not worried about putting the AC unit in the bottom, because even though cool air drops, it can't drop below the floor of your chamber, and hence, the entire chamber would get to the right temperature. The other issue you may have is where to stick your temperature probe. Some brewers advocate using a thermowell to get the probe into the wort. Others tape it to the side of a bucket or carboy. If you leave it in the "air", you need to make sure that the sensor is not too close to either the hot air coming out of your heater or the cool air coming out of your AC unit.
These are things I was struggling with when thinking about building a chamber like yours. I may just end up getting a used chest freezer on CL, and adding only a heater to it. Right now, I have a space heater inside of my extra fridge, and the heater is on the top shelf. My brews turn out well this way.
Nice project. Hey, quick question - the STC-1000 is only Centigrade, correct? Any body know of a device like that that shows Fahrenheit? I'm using an enclosed two-stage controller, so I can't build it into the frame like you are doing.