TBrennan
I have done quite a few door handles over the years. I am not completely familiar with the new 03 style door handles, but have done dozens of the older style. There are countless products on the market to make paint stick to plastic. I have always used Sherwin Williams finishes at the shop I work at. The steps taken are to wash them in solvents. One is like soap and water, but way more expensive, the next solvent is alcohol based and helps remove static that will cause problems. After this is done you scuff them lightly, tack them off with a tack rag and put a 1k plastic adhesion promoter on. If you have a textured handle, the next step would be a 2k primer surfacer. This will fill voids and texture to give a smooth finish. After the primer is cured you sand with 400 grit and then 600 grit wet or dry. Then comes the best part, color. With door handles, which arent a flexible plastic you wouldn't need a flex agent. This is only for the fascia and bumper parts where the paint may flex when an impact occurs. Sherwin recommends a ounce of hardener per sprayable quart of basecoat for all plastics. This would be the same hardener you use in the clear. After the color is applied and you are happy with the coverage and color you apply the clear. Note this is painting in a nutshell, and there are many steps I left out so my post won't be a mile long. Any specific questions, Ill try to help you out the best I can.
Derek