Heat pump with A/C. With my shop, after spending my life working in dark caves that were either freezing or broiling, I insisted on 2 things, very good lighting and HVAC.
Your building needs to be well insulated. In a hot, humid place like NC, I would think A/C would be really helpful - heat wouldn't be hard at all. One summer spent in a climate controlled shop will be like heaven. Stuff doesn't rust and you don't suffocate from the heat. Makes working on your junk more enjoyable.
My building is 40x52, insulated and is unbelievably cheap to cool and heat. On really cold days, I use a small propane space heater to warm it up to keep from beating on the heat pump too hard. Split system - outside unit with the air handler on the inside elevated so I didn't lose floor space.
If you spend any amount of time in your shop, it is money you will never regret spending.
