I have a small Danby bar fridge (no freezer section) that I converted to a kegerator.
I use a Johnston A419 controller on it to bypass the built in thermostat because I wasn't that happy with the original. I secure the sensor probe to the side of the keg under a bit of foam insulation with a flat bungie cord and keep the temp set at 39 F.
Works fine and I have not had any defrost problems - defrosted maybe once a year.
I also have a couple of small computer fans running in the kegerator - one just circulates the air and the other blows air via a one inch ID hose up into the beer tap tower to keep the beer lines cold.
Brian
PS - I have recently bought a few STC1000 two stage controllers on Ebay - more versatile than the A419 as they can control both the a fridge or freezer compressor and heater, and also a lot less expensive. I have had zero problems with them, but i do use a secondary relay with them because the contacts are just rated at 10 amps. I use them to make a freezer into a keg storage unit or a fermentation chamber.