If you're going to put central air in as well, then I would install a package unit, that has forced air heat and air. These sit outside, so they take up less room in the garage, and you duct them in. Put the cold air return down low, and you can put the deliverys in the ceiling or whatever you want. It will be easier to install, and will take up less room. The best part is not having to install a flu pipe, or run gas into the building. Thats how I'm going to do my garage, when I get off my lazy butt. You know how it is, I do heating and air conditioning for a living, so naturally, my garage has nothing in it right now. Kinda like the body shop guy with the rusted out primered car.