In general, a wider wheel like yours ought to be fine, however, especially with big 6-piston brake kits, the wheel's offset and even its design can play a huge role in fitment. That also being said, a wheel with a larger diameter would also be ideal. Your best bet is to give us a call at 1-877-578-2433 so we can get more specific about your wheels.
As far as other upgrades, we'd strongly recommend grabbing a six-piston kit for your front brakes. Throwing a heavy-duty six-piston brake kit on the rear drastically changes brake bias and your Mustang's general braking dynamics, and you'll need something more powerful up front to compensate. Again, give us a call at 1-877-578-2433 and we can discuss brake kit options for your vehicle.