Sorry for hijacking a thread, but:
but your looking at 4.5kg for motors/gearboxes if you use 4. and what speedcontroller are you planning on using? as each motor can probably pull over 150 amps each (stall). Ats all well and good having this much power, but can you get enough traction?

IFI Victor 883s and lots of skid marks.
http://www.toowoombatanks.com.au/videos/highlights.wmvhttp://www.toowoombatanks.com.au/videos/highlights.wmv
Not a very good video, but you get the idea. (and of Philips flipper Marauder.)