need help with getting a dependable setup

I don't think that a fan is going to cure all of your woes. 40 mph on the sand is a very tall order.

If you got rid of the two speed and geared for about 30, I think that would be a more reasonable demand of the system. Even 1/1 scale monster truck tires have something like "do not exceed 35 mph" stamped on them. The ones I've seen did, anyway.