I used to have a craftsman 3/8" power drill that I used for sanding rust off autos. It was a good drill; lasted me 12 years. The only thing I didn't like was it was heavy and often difficult to use when holding it at an angle. It was also very powerful.

What is a good power tool for sanding automotive rust off vehicles with a lot of contours such as the early 300ZX and Mazda B4000? I was thinking about the Dremel 4000 but that's also too heavy and a lower model number Dremel may not be powerful enough.

Dremel 4000 is too heavy? Weighs maybe a pound! You'll spend forever using a Dremel anyway. I'd use an orbital sander with a flexible pad or a 6" orbital angle sander.