As I see it, these things are the most important:
- Keep mass as low as possible
- Eliminate as much friction as possible from the wheel/axel interface.
- Make it as aerodynamic as possible.
That is a very good list. Keeping it light was the first thing that came to mind before I saw your list.
I would also suggest some things to consider, in no particular order:
Keep it more low-slung than tall, so that it not top-heavy.
Use a long enough wheelbase (distance from front axles to back axles) that the model will not tend to veer off to one side easily.
Try to make sure the wheels are as perfectly aligned as possible, so the car will roll as dead-straight as possible. Be prepared to be able to tweak to adjust that after you do some roll tests and may find it goes to the left or right.
Make sure the rocket engine mount is perfectly centered to the body (in yaw), so that the thrust will not tend to make it veer left or right.
The lightest and aerodynamically slickest car will not win if it veers off to one side, out of bounds (or however the contest is set up). So keep in mind it has to be able to run straight too.
It also has to be strong enough to hold together, and not break too easily. This is where trying to make it light can sacrifice strength, so you have to be careful in the design approach. I feel comfortable giving some generic advice but not to actually suggest a specific design.
BTW - some of the above about alignment presumes the car is free-running and not using a tether line. If a tether line is going to be used, then some of those I mention will not be nearly as critical as with a free-running car. But top-heavy is an issue regardless (you do not want it flipping over), and the more it tends to veer to one side, the more friction there will be on the tether line guide lugs.
- George Gassaway