Hi Sue,
Great idea and good way to motivate people to stick with their workouts. I think it all depends on what your competition will test. It may be a better idea, instead of making the competition multi-leveled, to really tighten the focus and make it based upon one set goal (either in the cardio or strength training arenas). Once you have reached a winner, you can migrate to another. I think this will be easier to track and will keep the competition really focused.
Great idea and have fun!
Good Luck,
Paul