Marine Grade Plywood?
"A catboat style boat could easily go up and down rivers, couldn''t it?..."
Like many of your questions that is a less than simple one. In a general sense, a well designed catboat could work reasonably well on a large-ish lake or wide river. I have added the modifiers because in a general sense, catboats do not go upwind very well and generally do not do all that well in light air. When you add the additional drag of heavy displacement and a full length keel, I would suggest that a catboat would be a poor choice. Here''s why I say that:
River and lake sailing tends to involve a lot of upwind work, because the wind tends to bend down the long axis of a lake or river. Lake and river sailing tends to involve a lot of light air sailing, punctuated by bits of very heavy stuff often in very confined quarters.
Because a catboat is stuck with a single sail, and because it is a tricky design expercise to design that sail so that it can be easy to reef and still maintain a balanced helm, it is harder to design a catboat that has enough sail area for light air and yet is not overpowered in a breeze. A boat that does not have good light air ability will miss a lot of otherwise pleasant sailing days which given the short sailing season in Michigan could be a real problem. At the other end of the wind range, because of the confined nature of most river sailing, being able to control the boat in a breeze becomes especially important.
In that regard, a more moderate design, with a sloop rig and a lighter displacement to improve your ability to sail in lighter air and upwind probably will work better for you than a catboat.
"Also, finally... am I in over my head with the idea of building a 20-25 foot boat? "
I don''t think that it is for any of us to say whether you are in over your head. I have been involved with a lot of boat building and boat restoring projects over the years and frankly it is never easy to predict who will succeed and who will not, especially without having met the person.
I am not sure that this is the proper forum for this kind of discussion. Please feel free to email me directly if you would prefer.