Dirtdigging101:

It can be done with a lower temperature thermostat applied to a stock window shaker, but I think you are going to have to have the coolant balanced to defer frosting.

Placing a heating unit in front of the coils for regular defrosting cycles would require a timer and preferably a radiant heater that could cycle when necessary.

The problem with that is you can't time it according to the frost buildup because of operating and humidity variations.

The best you can hope for is to overfill the unit with its recommended cooling fluid and set it to a 36 degree vaporization temperature at the evaporator.

Fairyprincess

How are you going to keep the humidity up ? the window shaker will pull out all the moisture and dry up your produce. you will also have to remove the stat from the window unit because the window shaker is set up for high temperature. It will have problems going down to 65F.with the original stat. It will work to a point. You need a system set up for medium temp. between 55F. and 36F. you'll need a low pressure control fitted to the suction line of the unit to control temp. after you have removed the built in stat. You could also go to an auction of a restaraunt renovation and buy the proper unit. It will be an R12 system so be carefull. Good luck.

The refrigerent probably does not make a difference for a medium cold room. This is assuming that you do not want to freeze the produce. I would probably install 2 inch foam on the walls, and use a 1 1/2 or 2-ton unit. For sanitary reasons, you need a moisture barrier over the foam, like hardboard used in bathrooms. The problem with the window unit is the spacing of the coil fins. They will freeze up quickly, so the system will need to defrost more. This will double the operating costs, and kill the window unit quickly. I would look around for a used system from a store, that can be installed properly. By the time you modify a larger window unit, you can probably spend the same money for the correct equipment. The difference will be in installation costs.