Will these motors are High RPM (5400RPM) depending on the size of wheel u use will depend on the final RPM ur looking for out of ur gearbox, 15mph is plenty of speed
because of the high RPM you will be able to gear it down enough to use in a heavy, if your going for 4WD maybe 1 motor a wheel,
ignore the recommended controller rubbish, if that was true for every motor people wouldnt go above 40A on the Bosch GPAs as thats they're recommended ampage too, rubbish! we've seen our boschs pull 100A on start up

put them on a good controller (which im sure you will) and ur robot will fly