That's a hell of a deal on great little heater. Cheapest lately was $119 on sale at Northern Tool and maybe $109 with another RetailMeNot discount.
Get the optional hose to connect to 20 lb BBQ bottle of propane.
I love my Big Buddy and it's saved my **** when sleeping in camper when out pheasant hunting in cold/snow. I used it in shop also sometimes and never have problem with fumes. The built-in low oxygen sensor will shut it down if there is problem.
P.S. OP you might also want the optional AC/DC plug so you plug into 120v and run the fan that generally takes batteries. With it running full blast at 18K Btu and the fan . . . I'd say an insulated garage that size will heat up within 20 or 30 minutes, or less. You could cheat a little and hang tarps (ceiling to floor) around the car you're working to keep the heat more localized. Once you've got the area heated up, you can back the heat down to medium or low.