Hi,yes I would 600 grit to get the boat ready to paint and yes ask your local autobody shop to do it.Autopaint last and take a better beating then spray paint and super quatily finish too.
I dont know how much they charge over their put it should be under 50 dollars to do it.Ask if they have any left over paints that they have in the shop.You might find a color that you like and the cost would be cheeper.99 out of a 100 the left over paint if they dont us it they throw it out.[>:]