The choice between marine and exterior enamel is not too tough. Marine enamel will stay shiny longer and require repainting less often, but at a premium price. I am not familiar with the Heron design, but for most home-builds I reccommend ordinary exterior eanmel. If you do not like the performance of the paint, in a year or two you can repaint with marine enamel and not be out much money. The opposite is not so inexpensive. No special primers are necessary for epoxy, just one that is compatible with the topcoat paint you are applying.
Hope for the best, but plan for the worst.