If you're going to roll it on you'll need the one as a kit with the catalyst MEKP.
Canary Yellow Brushable Gelcoat Gallon Kit w/2 oz. MEKP. - 137981
You'll need a gallon, you won't get enough down with just a quart. Don't worry about sanding down to the fiberglass in areas to get it smooth, gelcoat will cover it fine. I used marine tex to fill anything that wouldn't sand out smooth. Die grind out first, fill, let it dry then sand it smooth, gelcoat will adhere and cover it fine.
My experience if you're going to roll it on, do a section (say left vs. right side) and a pint at a time, and go at least 3 coats. You've only got about 10 minutes to get it down after mixing in the MEKP. You'll need at least 3 coats because of all the sanding to get the pitting out later. My trial and error experience, put down enough or you'll start going through later, watch the corners and edges, don't be as aggressive.
When wet sanding buy yourself a ~$25 2" air sander from Harbor Freight, set at 50 psi and it'll run all day. After you're done applying gelcoat and ready to start the sanding, it's the initial wax removal by hand using 150 grit, that's the pain in the ass part. Once you're past that part it's easier, because the sand paper isn't gumming up all the time.
I like using 150 grit pads on my 2" sander when prepping, it's pretty aggressive for the most part and it's fine for the final prep before applying gelcoat. Wipe it down good with acetone just prior to applying gelcoat.