+1 on the gel coat work before sanding idea.
Gel coat goes on amazingly thick.. anywhere between 35 and 50 mils, sometimes more. For comparison, most paint goes on at around 3 to 5 mils. So you can spend a pretty fair amount of time working with a 600 grit paper before you do any real 'damage' to the gelcoat.
You might consider - and keep in mind, i'm just some nimrod from an internet forum - simply starting with a 180 grit sandpaper (use light pressure, don't rub 'over' edges, and keep moving around) for the really ugly stuff, if there is any. Then move to a 320, do until everything looks about even. Finish with some 600, then get a standard automotive buffing compound, your favourite drill, and buff away. I'm always surprised what an afternoon of sweat can do for gelcoat.
Failing that, paint is always an option. A little fairing compound (think: fancy bondo) is easy to apply, can be sanded down to remove any dents/scratches/blemishes, and then painted over to make a shiny hull like the one painted above. But paint is not cheap, and if you are thinking of doing it yourself expect to finish the project wishing you hadn't started it. I know.. everybody says they enjoyed it. I think they are lying, based in large part on the delusions suffered while enduring the painting experience.
Guess who didn't enjoy painting a sailboat by hand?
Anyway, that's my two pennies. Which is over inflated, even adjusting for current inflation.