The rest of the body will be mounted on either a tricycle or 4 wheeled toy car (think Power Wheels).
From experience, I can tell you that using such a platform will be a challenge in itself; not because it is difficult to power such a machine - though you will need a fairly high-amperage - around 30A - h-bridge to control the drive motor(s).
The challenge is in steering it - or rather, coming up with a means to steer the wheels. That said, you can easily throw money at it and get something to work (ie - by using a high-power geared servo motor, like from Servo City - or a linear actuator from say Firgelli Automation); sometimes, even throwing money at it won't help, mainly because such ride-on toys weren't designed to be controlled in such a manner - finding mounting points for the steering actuator (or building and attaching a bracket system to hold such an actuator) can be a real brain-teasing exercise!
I can't offer you any specific advice, only a warning: Your best bet is to research how other people have solved the problem; for instance, if you decide to use a PowerWheels or similar ride-on toy, visit the Modified Power Wheels forum (http://www.modifiedpowerwheels.com/forum/
) and research how people have modded theirs for R/C control and similar. There's a ton of different solutions and ideas out there; both in forums like the above, and independent designs. Because there are so many different ride-on toys, manufacturers, and designs, there generally isn't a perfect, one-size-fits-all solution to the problem (unless you are able to purchase an identical ride-on toy that someone else has modded in a similar fashion).
Lastly - and this applies mainly for any homebrew servo solution (where you build your own servomechanism and/or linear actuator) - be sure to install limit switches for end-of-travel signalling on your steering mechanism, so that the motor will stop turning once the end-stops are reached. In my own case, I managed to destroy the teeth on the output gear of one of my gearmotors by not doing this...