I've been considering these little heating and cooling units for my dog house, would probably work great for a garage. It both heats and cools and keeps a constant temp. I've also seen them wall mounted. I found one on Amazon but I've seen cheaper ones:
A garage would need to be insulated to keep the cool or heat. Also a 8' rocked or drop ceiling would be needed, if its a old "A Frame" you wont be able to cool or heat it what so ever.