The volume of primer/paint you will need will vary depending on who does the application, how its applied, how thick you want each coat etc.
The more you buy the cheaper these products get. In my past experience, the price of two quarts may equal or be close to the price of a gallon.
Spray application is by far the best way to do it, better distribution, more even coverage etc. and much easier to sand between coats.
Primer helps the paint
to stick and will fill minor surface imperfections. A "high solids" primer works best if you have a lot of minor imperfections.
After you have made any repairs necessary to the fiberglass, sand the entire unit with successively finer grit paper. I start with 120 grit.
Any imperfections that don't sand out are further repaired with glazing putty and sanded with the 120 grit. While paint will adhere to sanded fiberglass, a light
coat of primer may help to identify remaining surface imperfections.
Sand the unit with 240 grit and repair any surface imperfections that don't sand out as before.
Sand entire unit with 320 grit.
Wipe exterior surface down well with degreaser.
If you are spraying you are done.
If you are rolling or brushing, you likely want to fine sand again and apply successive coats. While it is possible to get a smooth shiny finish using this method it is very labor intensive and takes much time. A thick coat of paint is required as you will be sanding much of this off prior to the next coat.
You can also sand the final coat with 1000 or even 2000 grit water paper if you want a professional looking finish.
Good preparation is 99% of a successful paint job and imperative for a good finish. The paint seals the surface and provides even color. Think of the paint as the final "cover" of the work you have done.
I suggest you buy a quart of primer to begin. You can buy more if needed.
I suggest you buy a gallon of paint. If you buy quarts, mix them all together in case there is a color difference between cans before application.
The first step is always to wash your unit well with soap and water and then wipe down with a degreaser. Otherwise, you may be grinding dirt, dust, wax etc. into the surface to be painted which may impair paint adhesion.
Some shops will spray your unit for a minimal price if you do the preparation.