3mph might be too slow, not enough wheelspin makes you rely on your tires getting a perfect hold. Not saying it's not good for your application or is bad, but my Clod with dual Integy 55t motors and 9t pinions hits maybe 6mph, and it just barely has the wheelspin to get up stuff. Granted, it's still hard for it to get stuck, but if I were you, I'd go with the "high" range.
The outside of the case is about the same size as a 540 motor can. It's gonna be a whole lot smaller than an emaxx or other transmision. I'm gonna put two gears the same size out the end of it so I can have the driveshaft run parallel right under the motor and planetary to the front and rear driveshafts.