Just curious, but why don't you think it will buff out? And what are you looking for? If it is below the water line you're not really going to be able to see it. It may look a little stained or something, but many bottoms do. Unless you want to repaint the sides up to the light gray stripe, or re-do the sides entirely, I wouldn't worry too much about it.
Many people on here have had good results with Easy-off oven cleaner for removing bottom paint. I'd give that a go first, and try to rub it out (personally I'd pay someone to rub the bottom of the hull out, I HATE that job, but to each their own...). It will be much cheaper and at least you'll have a starting point.
There are several quality automotive finishes that are fine for the job though if you're unhappy with your results. PPG concept seems pretty popular/cost effective paint that would take care of it.