You want enough linear force to break traction at stall, in a pushing match. Traction on the floor is pretty terrible historically though. Soft foam filled tires are your best bet. Relying on pneumatic is a dubious proposition, of course.
Bldc motors function like pmdc motors with respect to torque. You should be able to calculate stall torque from the estimate of max powe at 50 percent maximum speed.
http://www.linmotors.com/_images/BL16B17-08_NEW_HR.jpg
Check out the riobots manual if you haven't already. Really good resource for how to think about a winning design.
I really liked the new battlebots series, but that's an "only" hobby. Brushless tech has come SO far since my initial interest in combat robotics, where big pmdc motors like magmotor and eteks dominated. The power to weight and efficiency on the motor you linked is excellent and 2 of them would motivate a 100kg bot very well.