Late September weather may be quite cool. The elevation of the Grand Canyon plateau is like 6700 feet above sea level. Its even possible to get snow at that time of year there. Now granted, I-40 is fairly flat with no real bad grades, some mild ones around Albuquerque, but nothing too bad for the most part. If you all really like the cold/cool weather then by all means go have fun. Research things to do along I-40 because there are a lot of them and you will have to be selective if you want to reach your destination in your time allotted.
