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650b seems pointless here... With room for 26x2.0 or 700x42 WITH fenders depending on which versin, clearance doesn't really come into it. Dropping the 700's already low BB might cause some problems too. Cheap 26/29er wheels work fine for their respective versions, cheap or expensive tyres available in both to cover pretty much all conditions (lightweight 26x54c Compass coming shortly as an example...)
If its disc there is no reason not to; it means you can get cheap MTB wheels and put on nice and supple compass tyres for road usage OR knobby for off road ..