They offer them in a 15x8" as well from what I just searched. I would go for the 8" wide rim and appropriate tires from there. A 12" wide tire will be a bit much and can add a lot of unsprung weight and resistance to the steering system. Although that set up would look killer it may be a bit of overkill. As far as running street tires only. With a good alignment it should ride pretty good. As good as the shocks will allow.
Also it may take a little playing to find the optimum tire pressure to run. A good way to find best contact is to find an area with somewhat packed dirt to drive over then look at the contact patch. I would think with the tires you described a good starting pressure would be 22-24 psi.