Some OT
When i started thinking about my bot.... (it 6kg but its not so much smaller than a fw)
Two drill motors: ~£0
Two 'green' wheels: ~£0
35/40MHz RC gear: £5
Microswitch speedo: ~£5
Batteries from drills: no extra cost
everything else: free
HDPE (as an example) for chassis/armour: £1 (free leftovers)