yes the monotubes are worth it. the difference is that the twin tubes are crap.
you would need new wheels unless you run wheel spacers. 3.75" or 4" backspacing is good. and yes 15x8 if you get new wheels
i have BFG MT's and ran them in the snow last year and they did fine. Last year there was a part of the road that was dry from where tires had driven and i would put my tires in the ridge of snow in the middle and it jerked a little but nothing big. (of course if the road is all snow this won't happen.) when it was solid snow, they did fine