[EricH]

That's a bit better. 6WD colsons will probably work (and if it doesn't, you might be able to find some Lunacy wheels on AndyMark and use those--they're slick enough). That wide/long of a robot, you can probably get away with "flat" configuration, though I'd actually suggest using "mixed wheels", with at least one end being smoother/less "sticky" than the rest.

A couple of other items to bear in mind: I'd go single-speed transmissions, geared for a "reasonable" speed (say about 10 ft/second or less), but with two motors on each (in case of one failing). We're dealing with foot traffic here, so precision driving could be important. Also, some form of safety system should be on there, say a drivebase lockout (again because we're dealing with lots of people).

I'd agree on the superstucture being more "precision" than not; after all you don't want it rocking onto some potential customer's head while under the control of your sponsor.