Dave,
Interesting project, and a neat design.
I have built a few small boats, some from designs in Wooden Boat, like the nutshell pram and the acorn skiff. They are really fun projects. You have to take your time and do it right to get the best result, like anything else.
For materials, I would think on this project, you want it to be really lightweight, so I would lean toward some thin (say 4mm) okoume plywood and cut the side planking out of that. Maybe go a little thicker for the bottom if you are afraid of having to drag it around against sticks and rocks until the wheel takes the weight, or just use the thin stuff and be aware of the need to be careful. You could always add some guard strips on the bottom, using some oak or something tough and flexible like that.
The frames that support the sides can be real strong but light by laminating them from thin strips of wood that is steam bent to a form prior to gluing up in a jig that lets you clamp all the laminations together. This boat looks like the same contours might work for all the frames, so maybe you could just reuse the form and jig for each frame.
I think the bow and stern transoms would need to be a little thicker than 4mm, to give more edge surface for the side planking to attach to...
I'm sure if you got the plans from WBoat, they will specify some materials, and give good instructions on setting up jigs, patterns for the planks,etc.
One of the contributors to WB shown in the issue you reference, Greg Rossell, wrote a really good book on building small boats. There are a lot of good techniques in there that help a lot, and some homemade or modified tools that do just what you need to do.
Be very careful if you change any dimensions; not only is the width critical to stability, it is a part of the displacement and therefore the load carrying ability of the boat!!
For example, one of the easiest tricks I have found is to use self threading sheetrock screws to pull the edges of the plywood planking together while the glue is setting. After the glue is set, you back out the screws and fill in the holes with epoxy or bondo. This works especially well where it is hard to get a clamp to bear.
I think you are on to a good boat for the purposes you describe. Please post again with photos if you follow through! Good luck!