Concrete floor, 2x4 walls with foam spray insulation. Use skin of choice diamond plate etc. You can use a window unit with a modified t stat and an air defrost timer. A refrigeration system is a refrigeration system the way its controlled is whats different.. Most walk in coolers are r22. A freezer you can not make with a window unit. I reccomend finding a r22 window shaker as most new units are 410 and well I have not tried it with 410. To maintain 35/40 degrees is easy for a window unit
Hvac is my living. And we have a 8x8 that I built at the farm for butcher shop. Works great.