You can get some dents out fairly well with a slide hammer style dent puller. Basically, it has a screw on the end of a smooth rod with a sliding weight. Screw it into the dented sheet metal and slam the weight outward to exert a pulling force.
Another option is the stud welder, but it's more expensive. It temporarily welds short rods, or studs, to the dented metal (kind of looks like a porcupine). The dent is pulled out with the rods, then they are snapped off.
Sometimes, like on the door, you can gain access from behind through holes or openings, and use pry bars to get out the bulk of the dent. You may have to get creative with shapes and sizes of tools you use (tire tools, galvanized water pipe, wood closet rods, etc.)
Jerry