Medium grey sealer would be fine. SPI grey epoxy reduced as a sealer would work well. As for paint, basecoat it depends. If using a quality basecoat that reduces 1:1 then 1-1.5 gallons (unreduced). Clearcoat will depend as well. How many coats are you planning? If you are planning to cut and buff plan on minimum of three coats, four would be better. Using SPI Universal, I would plan on 1.5 to 2.0 gallons (each) of clear and activator. If you are going to use epoxy for your base primer, minimum 1 gallon, more if you use it for build. If you aren't then plan on either 2K urethane or Poly primer as well. You will also need at least 2 gallons of reducer.
Don or 68 has a formula worked out on applying base (?) and clear. I think they figured 8oz per panel, per coat. Maybe one of them will weigh in on if that is correct.