I use to fit 4 wheels+tires in the back of my old Bronco II, and that thing only had 1/2 the space as the MO does. So I'm willing to bed you can do it without even putting the back seats down. But if you have to, put the back seats down and you'll have tons of space.
No, I don't like any of the 20's I've seen. I'm going with Racing Hart Evolution GT5s, 19x8.5, 35mm offset, Silver painted. I'm going to use Pirelli Scorpion Zeros, 255/55R19 for summer and probably Goodyear Wrangler HPs in the same size for winter.