Epoxy or Urethane High Build... what's next for me?

Just keep in mind that even though 2k and high-build primer sealers are catylized, they still shrink significantly over the years. My point is that while they are very good and have their place, using less(aka a thinner mil thickness) is always better. In other words, get your sheet metal as straight as you can, so you don't need too much product to get your panels "flat".