Did you decide what you are doing about heating the garage? The simplest option would be to add the garage as a separate zone to the baseboard heating. This may be overkill for your situation.

If you want a cheap option you can buy a heater that attaches to a propane gas tank. I have seen these in Home Depot. Another option would be to add a ductless mini split system. I think this would cost more than than adding another baseboard circuit.