First insulate the heck out of it, if it's not already. Don't forget the floor and roof.

I would go with a window A/C unit for summer, just note which way the cold air blows from it, so it's not over-cooling a rack. If you have the space available plant some trees so that the shed is shaded during the hottest part of the day.

During the winter I would remove the A/C unit and seal up the window, then run oil-filled heaters in the shed.