Yes, you can safely live and travel using a car if you choose too however it is never going to be as comfortable. If you plan on using a car the strategies outlined in the book are even MORE important to you since you will need more help in certain areas. Yes $600 per month covers food, gas and lodging.
As for the law and whether it is legal; laws differ from place to place but the book shows you how to get access to thousands of places where it is legal to live and sleep in a car and most times do it for free without so much as a moment of hassle. I have a chapter on getting your mail in the book which covers this tricky issue in great detail. My mail strategies make it so you can have either a po box OR a regular street address and still get your mail forwarded to you WHEREVER you are in the United States!