On the cheap vs. full marine--there is a middle ground. Around here, at least, big box stores (Menards, Lowes) sell decent 1/4" Baltic birch plywood. By "decent" I mean very smoothly finished and pleasant to work with, 5 thick plies of equal thickness, and no voids I've ever hit. Last I looked it was about half the price of marine ply.

The trade-off is, it's heavy, and is not marine-rated. That said, I have sailed two boats built of it for 11 years now with no sign of problems. I think it's perfectly fine for a dry-sailed trailer boat.

And did I mention it's half the price of marine ply?

As for epoxy, you could start with a gallon and see how far that takes you. That's where economy kicks in. If you get near the bottom of the jug and see you'll need more, easy enough to order.

There have also been boats built with PL Premium, which would probably work with a design like the Mayfly. But I'd rather use epoxy.