If you are new to robot building, consider starting with an off the shelf robot chassis complete with motors and wheels and like the rugged aluminium MiniRover 4wheel drive chassis from Microbot. If you wanted a tracked vehicle the Dagu Rover 5 comes complete with motors, and quadrature encoders for both treads. Prefer legs to wheels or threads, try a Phoenix hexapod kit which includes brackets, legs and hardware, or make a biped with a torso kit and add aluminium brackets and motors for the limbs.
If you require a small robot the Pololu chassis kits are ideal and are designed for use with the micro metal motors or Tamiya gearbox. By adding a robot controller and a few sensors or RF modules of your choice you will have a great mobile robot. Sintra boards are PVC sheets which are useful and easy to use if you prefer to make you own design of chassis. They can be glued to make a sturdy structure.