On the other hand, if you are not going down to bare metal in all areas, use the current layers of primer, paint, etc as your substrate. HAND sand using sanding blocks of different lengths using 80 grit to start. Once its very uniform (not in color, but in smoothness) use body filler to smooth those low areas, then hit the whole thing with epoxy primer.
What you are doing is rather than spreading a 1/8" skim coat on everything as most people do, you are using your current substrate as the first round of filler. I have done several entire trucks and have no mechanical, electrical, pneumatic sanders at all. Once you master the art of the feel of the work, you will be able to fine tune your technique. At that point, if you feel mechanical tools are still necessary, get out the wallet. That was my plan years ago, then discovered no need at all to get the tools!
There are other considerations. For example, anywhere you apply heat for welding up damage, your substrate will be compromised and you will need to sand to bare metal in those areas. I don't keep any old substrate myself, I sandblast everything, but many people do the above technique with dazzling results.
