Get a radiant tube heater. Its your best bet. The difference between them and forced air, is they heat objects. My shops drops down to 50 at night when I turn them down. I can also get my shop in the mid 60's in 20 minutes. Once vehicles are warm the heat will maintain itself in the shop by radiating back off all metal surfaces. I work in the 60's in the winter, but if I need it warmer, it will bake me out by turning it up.
If you can afford it put a wood burner outside, and pipe the floor.