What exactly does maxing out the cash counter unlock? By "Free Garage", do they mean an actual place, or are certain items free in the garages?

I'm not sure, but to the best of my knowledge, you don't get anything if you max your currency; I think it is a mix up relative to a cheat which gives you unlimited moneys as when you do this, your money indicator maxes out! Hope this helps (and is right), be sure to tell me if I'm wrong!