While you are looking at getting a good air conditioner, don't skimp on the insulation. I spent $1000 to have my garage spray insulated with cellulose. They sprayed the sticky stuff in the walls before sheet rock and the loose fill on top of the ceiling. The best part is that I didn't have to deal with pink crap.

I have a 20X24 detached garage that I keep cool with a 6000btu window unit and it keeps up fine in Virginia summers. I know that the unit isn't rated for my square footage, but it doesn't struggle. I heat it all winter with a single portable space heater. I was going to install a minisplit, but I spend so little on conditioning that I'm not sure there is a break even point before I would have to replace the unit.