It's just a standard heater that heats and blows air, we call it a hot air system when it is installed in a house. As opposed to a hot water system that has a boiler that circulates water through radiators or baseboard.
If it was me, I would find a used hot-air heater someone ripped out of a house and hook it up on the cheap. Mini-splits are too much money for a garage in my opinion. A heater that burns gas or oil will also heat the place up a lot quicker than a heat pump.