My grandfather built a plywood boat in Grand Rapids many years ago before we had such good design and material choices. You want to sail in some wind and with 4 people on board so the CS15 sounds like the way to go. Its just a little larger, with a few inches more beam, a newer design, and designed primarily for sailing.
You might want to look at the CS-17 again if your going for the big water. Everyone always thinks they should have gotten just a little larger boat. But I think the 17 is not too large for most. One person can handle the 17's masts and it can fit in a standard garage or carport. A little harder to row and more cost of materials to build- thats true.
Enjoy the journey- Tim