At some point I could probably do that with a small room, heavily insulated. but that would be much later.
Could I do that with a wall unit AC
first I want to keep it between 68 and 72 and in the winter less than freezing unless I am fermenting.
Later on I will build an ageing room.
You could maintain 70-75 with a window unit and batt insulation in the walls and ceiling, maybe even a little bit cooler.
Where we live, with high humidity much of the year, if you intend to do 55-60, the vapor barrier becomes paramount to prevent condensation and the resulting mold and mildew. An oversized ac unit may serve your needs, but could cause excessive humidity in the room, as opposed to a thru wall refrigeration system.
Best bet, if you can swing the cost, and since you have to insulate the ceiling anyway, use closed cell spray foam insulation in the ceiling, walls too if you can. The closed cell foam is a vapor barrier and you'll be set for the future, whether you decide for room temp or storage temps.
Mike's right, a little electric heater with t-stat would be fine for heat if your ac unit doesn't have it.