Depending on the bottom width of your 18', the paddle jon would probably draw more water under power. Its impossible to know how much without having some idea of the weight its carrying. Its always hard to know exactly how much you're drafting, but with two lightweight guys and a little fishing gear, no motor, about 2" possibly a little more.
Sand is going to wear the impeller, no doubt. The new stainless steel units are better, but again, impossible to know a time. Different conditions give different results. I spent a lot of time on the Thompson, known as the Grand then, and I it seemed to me there were always channels that offered a little more depth. You wouldn't be "in the sand" that often I would think?