It's a decent idea. Options are great. You should beta test the idea before building ... perhaps go to your local shop and see if they have a Cannondale Slate, etc, you can borrow the wheel from.
A 650b x 42mm wheelset is about equivalent to 700c x 23, but note: don't try to match the diameters of 700c wheels. It's apples to oranges. It's more a matter of finding the right sweet spot for air volume and tire contact patch for the riding you want to do. Besides, you will likely run into fork crown and chainstay clearance issues especially if you try to run a very fat tires with knobs, and if you are running a wide rim.
If it's strictly for trail riding I'd personally stick with 700c x 40mm. That's plenty of cushiness, especially on a wide rim. You can adapt to pedal strike somewhat, but I think it's annoying to have to push a bike uphill because you can't clear the tree roots at a slow pedaling cadence.
To answer your second question plenty of us have run 650bx42 Babyshoe Pass, Grand Bois Hetres, and the Panaracer offerings, and they fit your criteria for decently light, fast 650 tires and then some.
Originally Posted by
3speed
I just got a new frame and it's set up for disk brakes. It's designed to take 700c wheels with up to a 700x40c tire. I'm building a set of wheels for it, and am considering going with 650b rather than 700c for the ability to run higher volume knobbier tires for trail riding. Is this a decent idea? Is it a terrible idea? Will I lower the BB too much? Negatively affect handling? Are there decently light, fast 650b tires out there for when I'm sticking to the road(probably in the 32mm wide range)?