For the pin removal, try heating the pin using the tip of a soldering iron. Be very careful not to touch the surrounding plastic as this could harm the figure beyond repair. First, figure out which end of the pin see threaded. Then heat the pin for a few seconds. Once its heated, push from the other end. When the tip pokes out, grab it with your needlenose pliers, and pull it out. Go slow, don't rush. If you melt the plastic too much, it's ruined.
As for prep, it all depends on the paints. For most of my work, I lay down a base of Krylon Camouflage Ultra Flat Black as a primer, then go over that with Testers enamels. You can sand it to promote adhesion, just be sure to use a 1200 grit paper or higher so as not to gouge the plastic. Also a good idea to transform the figure while in the primer stage to find rub spots and then sand those down using an emory board first, to remove the bulk of the material. Then use progressively high grit sand paper to smooth the finish. Go slow at this stage as you can always take away more plastic but, you cannot put it back.
I hope this helps you guys. Another word of advice, practices these tricks on junkers! That way, if you mess up, no harm done to your project.