Old Salt, I have a buddy that runs pretty much this same rig, has for about 10 years and loves it. He runs it around MT with the standard 2 fishermen + rower and does extended overnighters with his family (2+2 kids). I myself have no desire to run a cat but he adores his boat and I'm sure you'd be happy with that set up as well.
I also noticed you asked the buyer a question about Jack's (JPW). When you get a chance research them (him) Jack knows his shit!! and he sure appears to be a great, standup guy. He posts here on occasion (probably when not too busy building boats) and is always helpful and rarely, if ever is in salesman mode. You cannot go wrong with a JPW boat.
On size I would error on the larger side, bigger boats float higher with the same load and as you may know Montana rivers can get pretty thin by august. I just went from a 13' round boat to a 15'er and can't believe the difference in room and draft. Staying high in the water will increase maneuverability and reduce effort rowing for fishermen.