How to love sandpaper
The old saying in the auto body world is "sand, sand and when you are tired of sanding sand some more". An old friend that taught me how to paint ,and yes SAND, told me that the prep guy makes the painter. This means that the more time you can spend getting the body smooth, grease-free, rust -free, and straight the better your paint will look. In paint the small details count, if you don't see or better yet feel the details (with your hands) you might as well save your pride and pay some-one. When you think you have the body "perfect" look again and spend about 10 more hrs. It also all depends on how nice of a job and how much time you have to do the job.
If you have the body off now is the perfect time to sand and fix rust or dents that you want to rid the truck of. I would start with a file sander so you can find the dents. Always try to work the steel out with methods other than filler (bondo or other plastic filler)
As far as the bottom of the truck, inside of the fenders, inside of anything that you cannot paint with the body on paint before. Any thing you can paint with the body on do so. This will save from scratches and marred paint in reassembly. with the doors off you can roll the cab on its side and paint or under-coat the bottom or you can do this with the cab on stands but be careful
However I would not paint the panels off the truck, putting the truck back together will be a treat in its own (lining up seams an things) let alone trying to keep the new paint scratch free.
Lots of words and the surface of the auto -body and paint world have not been touched. If you have questions ask
CumminsKid