I will attempt to answer from a technical point:

1. What is the range of output voltages (assuming there are a range) or what is the steady voltage from the RX18 to each of the motors? Depends on the motor curve, most toy hobby motors peak efficiency about 6V

2. Is the power of the motor (and by that I mean torque) related to the voltage of the motor/voltage being delivered to it or the current (amps)?.
Amp draw will determine torque, more efficient motors have multiple armature windings and high permanant magnet structures or electromagnets

3. Is it possible to fit (or even buy) motors with very high torque? and by that I mean much higher than the stock motors.....
Yes indeed, look for the increased turns refered to as "T" in hobby motor windings for power (rather than speed)

4. Where is it possible to buy things like the cogs, wheels and bits and pieces used in the gearboxes or parts for chain drives (ie. small drive chains and the like)?
RC car and nitro transmissions have a wide variety of quality gears

5. Can a stock motor/gearbox combo be turned to 3:1 using a chain drive and appropriate cogs without dismantling the gearbox?
Yes, but takes up more space, unless you are mechanically inclined probably better to purchase a munufactured unit

6. Are there any threads ( I checked with no success from my side) that anyone knows of dealing with big powerful motors in 1:16 scale tanks?
Try searching for "lathe" or "55T lathe" type motors, lower speed increaseed torque for crawler RC trucks

7. Is there any measurement of strength (as in "pulling power" or whatever) that has come to be adopted by tank people? Like pushing a weight, or managing an incline or whatever?
Not that I know of but it would be interesting to have a"tractor pull" competition of tanks!