As lovely as the whole modular get others involved thing and use second hand materials is, if you want to have a good robot, design it and then find the materials rather than finding the materials and then designing the robot.

If you want to get a kit for fun with robots then give the rampaging chariots guild a shout. Nice controllable machines that can have additions made and not a bad price for em but combat machines for full on combat are a different kettle of fish if you want to get far.