[Rimrock]

I suggest to use more time studying building threads of similar sized loaders at the web before buying too much stuff. Problem 1 is often one does not get enough power on the wheels at first try, or the correct speed. Don't know how you are planning setting up the drivetrain, there are 2 options;
- 1 off gear motor on each wheel, like the 'Le Tournau' equipment. The axles are made of tubes, and the gear motors are installed in the tubes.
- 1 main gear motor and 2 off axles with differentials, like Tamiya truck axles, all linked with shafts/universal joints, like the RC4WD loader

If it's about 4-5'' diameter wheels, around 50-60 rpm is suitable. The gear motor you are linking to are weak, probably a 1'' diameter, not suitable in a loader. If going for 1 off main motor, you will be looking for 540 size motors, 37mm diameter. If going for 1 off gear motor on each wheel, there are some motors 28-32mm diameter that can be suitable. 37mm diameter is even better, but might give quite low ground clearance for your size of loader.
The voltage is like the pressure the current is pushed into the motor, like hydraulic pressure is given in Bar. 7.2 V and 12V is the standard in this hobby. Voltage is given in the description of the motors, same with ESC, then one choose a battery with the correct voltage and size fitting into the model.
Dual shaft motors is some specialized stuff, often high cost, mostly used for industrial purposed. One can hook up to the motor in both end of the shaft, and possible save some space when it comes to gearboxes etc. Forget about that for a RC loader. If going for a single main motor, one connect that motor to a gearbox, and the gearbox then have dual shafts, for rear and front axles