Are you SURE it won't go below 30 or so? How long does it stay at or near 30? To play it safe, you can get a 2-stage Ranco temp controller, hook the cool side up to the freezer, and hook the heat side up to a light bulb, brewbelt, heating pad etc that is in the freezer. If the temp in your garage only gets to near 30 for short tmes, this is just for peace of mind - it probably won't turn on much - but could save you from frozen beer!
What temp do you want to serve at? 36? 40? 45? Up to you.
Plan to do something for moisture control - it sounds like this is not a 'frost-free' freezer, and water will condense on the coils in there, and then drip down to the bottom - put a towl or someting in, as well as a container of damp-rid or something.