Using the card to make patterns for ply is just fine.
When you get to considering the line pull, think "tug boat". They rarely pull from the stern so that they can steer when pulling, they usually have a low rear deck to keep the center of pull low. Any boat needs an access hatch, on most models this is the superstructure, but it does need care to ensure that it is strong enough to stand being hit by a wave, secure enough to stay put, and have a very water resistant join to the deck.
Styrofoam is god at providing flotation, but does have the downside that there is less air volume inside the boat. Any water that does get in gets to the electric bits that much more quickly, so waterproofing them gets a higher priority. A search through the offshore racing threads should give some helpful information.