You don't say anyting about the hull. In the classic wood framed Klepper, much of the strength of the boat is from the interaction of the frame and hull. The frame fits quite tightly in the hull envelope and some of the longitudinal members are under considerable compression. It's a very tough boat. I've paddled mine fully loaded in some pretty big water and never had a moments insecurity (About the boat anyway. Me, that's something else.). Hannes Lindemann took one across the Atlantic. I don't think I'd try that with PVC.

So, if you want a proven, durable boat, the Klepper is pretty hard to beat. But if you want to mess around and build a cheap boat to have fun with, "how hard could it be?"

A couple of thoughts: I would think the thinwall variety PVC would be right out of the question. Second, the failure mode of PVC pipe can be pretty spectacular, with splinters and shards flying around and unfortunate jagged adges. Wood and now aluminum, the usual longitudinal frame materials, are much less dramatic.