The car does not need to be kept warm but you do when pottering in there. So no reason for 24 hour heating, or however long you keep the house central heating on.
On the other hand the car is sensibly kept dry so occasional while you work heating using a calor heater for example isnt the best idea. In stead you might use a fan heater or even a wood burner. As a further alternative, Lidl do an excellent line in insulated boiler suits that are easy to work in.
If I were rebuilding my own garage, the one thing I would install would be a urinal. Its incredible how often I start a job only to want to pee. Plus a sink of course.
