Hi,
A lot of 'rock crawling' radio controlled models use what are called 'lathe motors' I assume these are motors that would have traditionally been used in hobby lathes. If you look around any RC Store focused on rock crawling, you will find lots of DC motors with detailed torque and rpm specs. They can also be powered through electronic speed controllers, you could get a motor and speed control for around 30 Usd. There are also 2 and three speed gear boxes designed to work with these, put that together with electronic speed control and you have a lot of flexibility to handle different materials.
Some well priced motors and specs here -
Just and idea,
Duane B