That is nice of you to buy your own car to volunteer at a dog adoption agency. We rescued our personal dogs and they are part of family. But when have them in our vehciles they are without cages so they can be comfortable or stick their noses outside thr window when the vehicle is stopped while at a stoplight.
Do you travel with them each caged? If so what sizes are the cages and one of us with a larger vehicles could give an idea of what would work. Are you traveling on gravel roads that mean mud or small river beds? Do have hills that maybe be snow covered or have close to one foot of snow at a time?