Need advice/etc. from those with painting experience

OK, I've done a fair amount of airbrushing in small scale, but I wanted to try my hand at some larger stuff. Specifically, to paint the backsides of some aftermarket fenders, bumper, side skirts, underside of hood,etc., prior to installing them on the car. I figure they aren't high visibility areas so I should be able to do a fairly decent job.

Here are my questions.

I've heard that you can get premixed paints (ie. retarder, paint, etc. already mixed together) from most auto paint shops, my concern is telling them what I need. I know I need a primer, but do I need a couple different types? Some of these parts are polyurethane, and I know in the past the folks that painted my car used an additive that allows for the paint to flex slightly. Is this really needed for newer vehicles? Also, this is a metallic, so I know I will need a clear, but what type; polyurethane based paint, etc????

Is there any books I should look at getting?