There are plenty of rover kits that are designed to be custom modified as well as integrated with an Arduino. These kits will almost universally be a better option than buying a new toy and hacking it apart.
Now, buying an old RC toy car from a yard sale for a few bucks, that may be worth the effort, since there's little financial investment involved, and a similarly sized robotic rover will be substantially more money (think of the Dagu Wild Thumper 4wd/6wd rovers).