What is the proper procedure for spray painting metal and plastic?
I did spraypaint a computer case a few years ago. It was glossy and I could see my reflection in it, but not as scratch-resistant as I hoped.

Question 1: You need to scuff to remove the smoothness of the steel and plastic to give the primer a surface to stick to. No need to go back to the bare metal.

Question 2: It depends how smooth you want your final result to be. Your end-result will be as good as your priming job. Prime, wet-sand with 1000grit paper, dry, repeat. I did this until I had removed all the "orange peel" feeling to my surface. (Took 6 or 7 coats) I used store-brand automotive primer and paint.

Question 3: This process give the smooth finish. The more coats/sanding you do, the smoother and glossier it will be. I used 1000grit for the first 3 coats, and then 2000grit for another 3 coats of paint.

The ??? step:  


Apply a few coats of clear coating. That's what I didn't do back then. 
When dry, polish it with car wax.