The engineering process is always the same. First, quantify your requirements. How much torque needs to be delivered at the wheel/ground interface, in order t

The engineering process is always the same. First, quantify your requirements. How much torque needs to be delivered at the wheel/ground interface, in order to get your maximum load over an arbitrary obstacle like a stone or a tree root, uphill on your maximum rated grade? That requirement will tell you whether you can source a motor (based on the torque it's rated to deliver) or whether you need to buy a drivetrain or (RIP your budget) design a custom drivetrain. Accounting for the weight of the motor/drivetrain will require you to iterate on that first requirement. Welcome to the pain of engineering design.