What type of motors?

I have a difficult task in hands, spend money with robotics stuff. 8^)
Here's the thing, my new project involves creating an autonomous vehicle
based on a kid's ATV platform (like this one: http://www.racewayatv.com/Kazuma-SFX-110.php ). It is pretty much set in stone that we will use that as a platform.
My problem is that I also have to specify a set of other components in the same list, including motors, solenoids and motor controllers. It's kind of hard to specify a motor without knowing what kind of force it will have to pull, not to say that I don't even know where I will mount the motor or how I will transmit force from the motor to the handlebars/throtle cable/brake/shifting lever, so I appeal to this list. Maybe some of you guys have more experience automating something like this, so I'm open to suggestions.
What I'm looking for (if possible) is suggestions for a complete solution for actuating this ATV. Anyone?
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