I'm no heating expert but my guess is you only need a unit that puts out 10-15,000 BTU - and possibly even a smaller unit might do it.
If you want to go the inexpensive route then hedonite's suggestion of a wall mount furnace (LP or Nat. Gas) would be a good choice. Just keep in mind that if you get a used one or a really low end unit you might have a burning pilot light at all times - thus you need to be a little more concerned when you have paint fumes or other flammable stuff open in you garage. Many of the newer units will have an electronic igniter rather than an open flame.
If you have more money to spend then the infrared electric unit is pretty slick. No noise, no flame etc. and most folks report they work very well in a garage situation. They will very likely be more expensive to operate as well since in most areas of the country electrical power is more expensive per BTU than LP or Nat gas.
Also keep in mind that you really don't need to heat a garage like you heat a house. I have my shop thermostat set at 57 degrees and that makes things plenty comfortable for working with a light jacket on.