I can understand wanting to make the pseudo-walk-in, but from an efficiency standpoint, the freezer is going to be your best bet. One of the big problems with even an mini-fridge is that if it is not properly sealed, or every time you open it to check a keg, the cold air is lost. Versus a freezer where the cold air sinks to the bottom and hold it. My concern would be mounting the AC unit, and the amount of heat the individual unit gives off.
EDIT: This is one of the builds I looked at a while back, might give you some ideas.