I'd buy a new boat
Seriously, how many chances in your life are you going to get to hit the Grand? Get a modern river runner. I consider the RPM an old boat and my newest boat is a decade old.
Look for something with a planing hull with speed. It will allow you to get out and spin on some of that green glass and not kill yourself on the flats. I think I would look for a boat that I could catch little D on westwater with, and then have fun on that wave. When I bring smaller play boats, I frustrate myself trying to eddy into that wave.
I think if you brought a diesel, it would be comfortable, but you would want something with a little, or alot more ability to play. There are so many boats that fit between jackson play boats and a diesel.
Just my two opinionated cents.