Good luck with your Kickstarter project.
I'm pursuing a similar project, though bigger (and self-funded). I'm using a PowerWheels Hummer H2 for my base (ultimately, I want to have a final platform using a Polaris ORV - that will depend heavily on available funds at the time, though; I've contemplated just converting my 79 full-size Bronco - heh).
For something closer to the lines of the PackBot - have you seen the MGA Tarantula radio-control toy? I managed to find one (sans transmitter) at a Goodwill a couple of years ago fairly cheap. Unfortunately they are no longer made, but there is still a stock of them out there that people occasionally try to offload via Ebay and such.
For your project, though - for ideas and such vis-a-vis your platform - a good place to get ideas (though most of these are track based - there are some wheeled vehicles):
Another good place is to look at old back-issues of Popular Science (on Google Books) from the late-1950s and early-1960s on 6-wheel ORV machines that were popular in that period. There are many articles, and several with more than enough detail to see how they were constructed.
Also - Parallax used (maybe they still do?) to sell a small robot (about the size you are mentioning) that was a strange "hybrid" vehicle; no suspension, just a skid-steer four-wheel drive platform. It used a small gas engine coupled to a small hydraulic pump, and using solenoid valves, dumped the pressure from the hydraulic system (it was proportional control) into small hydraulic motors which were coupled to the wheels. No transmission or such, just hydraulics. It wasn't cheap - but it wasn't outrageous for what you got, either.
Finally - look up the various papers at Carnegie-Mellon University on the CMU Terragator platform - that might give you some ideas, too.
Good luck! 