For sanding you start with the coarsest grade until the paint imperfections are gone then work through the grades to the finest grade you are using. Each grade will remove the scratch marks left by the previous grade. I would start with 240 grit & finish with 400 grit, you are roller painting so you don't need a very fine finish. before your next coat.
You will never get a flat spray gun finish with a roller, there will always be orange peel. The closest you'll get is to apply the paint with a mohair sleeve then go over the paint with a good quality brush, using nice even brush stroke from one end of the panel to the other. This is called 'laying off', as mentioned before. You need to do this before the paint gets tacky so on a large panel split it into sections, laying off each section as you paint it.
You will never get a flat spray gun finish with a roller, there will always be orange peel. The closest you'll get is to apply the paint with a mohair sleeve then go over the paint with a good quality brush, using nice even brush stroke from one end of the panel to the other. This is called 'laying off', as mentioned before. You need to do this before the paint gets tacky so on a large panel split it into sections, laying off each section as you paint it.