Which door card parts? I've done some of this myself.
If they've got the 'Orange-peel' effect on them you may want to have them 2K primed, which is a chemically hardening paint. If not, for I'm guessing you're speaking of the speaker dressing, the door handles and the arm rest, a normal primer would be decent enough. Halfords do decent primers, and from experience the hard moulded components don't necessarily need plastic primer, because a plastic primer allows some flexibility in the component. if these are the parts for the door, then they should be rigid pieces.
Key the components with a fairly low grade paper, probably up to 180 or 240. Primer and key again, but to get it to a smooth finish (an idea would be something like a 3M superfine sponge and then rub with a mutton cloth). If it's a Halfords shop, pick up some Rubbing Compound.
When you pick your paint colour, make sure you have the correct base of paint with the base of the primer, if it's Acrylic or Cellulose. Halfords I believe has mostly acrylic paints. Get a decent clear lacquer for the finish too. Another idea may be to pick up some different spray cap heads for better coverage.
I know this seems like an idiots guide, but I thought I may as well explain an approach
Shall have my stuff up soon so can show you what I've done.
