Solved

BMI

How important is an orange BMI level? Will I have to train them more and spend more gold or would they be good if not trained a lot?

Hello,

Weight (and therefore BMI) will only change if you complete driver training.

Green BMI (of any value) is optimal and orange BMI is less than ideal. 
For the best speed, I'd recommend trying to ensure that your driver's BMI is as close to ideal as possible.