Changing the body color

Sounds to me like your first step would be to go see a paint shop before you do ANY sanding or priming of the car. I would definitely avoid doing it the way you are considering. A reputable body / paint shop basically won't touch a car that has been "sanded and primed, ready for paint" They will want to start over, sand off your work so they know that it has been done right.

Personally I like white cars! I'd leave it the same color!

How is the current paint? Any rust, dents, damage? If you are considering a repaint just to change color, it does not need to be taken down to bare metal if the white paint is in good shape. Your paint guy will advise you on this. If your white paint is fine, it just needs to be scuffed and then proceed to primer / sealer and paint.

I'm not a paint guy, just my experience. Good luck with your project.