That should be plenty. In fact, perhaps too much. The total force you need to apply to the wheels is:
Force = Weight*sin(15)= 60Kg*9.8 m/s^2*sin(15) = 152 N.
The resulting torque to apply to the wheels is:
Torque = F* 5in = 152 N* 5 in * 0.0025 m/in = 1.9 N-m
So if you have 4 motors each need to apply about 0.5 N-m of torque to get the bot up the incline. Now, you need a bit more than that to allow the bot to accelerate, and to overcome resistances in the gear train and friction of the tire on the ground, but of you have four motors each with 13 N-m torque you have really over-sized them.
In terms of horsepower:
HP = Force * velocity = 152 N * 0.22 lb/N * 5 miles/hr * 1.47 ft/sec per MPH = 250 lb-ft/s, or 0.45 HP.
To put this in perespective - remember that a 1/2 HP motor is more than enough to run an overhead garage door opener - so if you are thinking of four 3 HP motors your bot will probably hit 60 MPH in a few seconds! One motor will be more than enough.