1. As mentioned above, you really only need to sand it lightly just enough to rough up the surface underneath to allow the paint to stick. No need to go back to bare metal unless trying to remove rust or something. Mine had a few dents in it which I filled with some gap filler before sanding it back.
2. I didn't remove any of the plastic trims, they just got painted in place. I used a roller for the most part which gave a pretty nice even finish. Some parts around the trims or at joins in the metal I used a small brush just to get into gaps. I went for the roller since I've never spray painted anything and it seemed a bit easier and potentially less messy.
3 and 4. I didn't prime it first, and yes the paint I used was called White Knight Rust Guard epoxy enamel. It's a one step paint so no need to prime. I used satin black and gave it two coats. Don't make the mistake I did and forget to paint the hinges too. It didn't take much, I think about a 500mL can and I still have some left over. Your freezer is bigger than mine though so not sure how much you need. Just calculate the surface area you need to paint and then read the directions on the can for the coverage amount (remember to double it for a second coat etc).
5. Mine doesn't have a cord in the lid but from what I've read there should be enough cord tucked inside the freezer to allow you to get enough slack to fit a collar on and not have to cut it.
Hope this helps, I'm currently in the process of putting together a few upgrades to mine. I'm making a shelf for a drip tray and I made a wooden blackboard so I can label the taps. Loving it so far.