The magnets would have to be so strong they would be dangerous to work with.

The easier way is just weld rods on to pull a dent out with, and then chop the rod off. There are several dent pulling tools that work this way.

Is your car body magnetic? If you had aluminum body panels, it probably could, though the panel is probably a good deal stiffer than this tuba. If you had steel body panels (which is very likely), it wouldn't.