[asid61]

If they have no CAD or design experience, I would hold off on shifting gearboxes. They are complex and the design work is better left to experienced gearbox makers IMO. A single-speed gearbox would have a much shallower learning curve. It's hard to beat the weight and size of WCP or Vex gearboxes anyway (excepting 192, whose gearbox is the best IMO).
If you do decide to go with a shifting design, I would get it checked by another team or somebody who has done work on shifting gearboxes before.

EDIT:
1. 8lb chassis? What does that include? Wheels, bearing blocks, etc?
2. I recommend hex axles. Saves time and stress in my experience.