I have general business liability at $300K coverage and the garage policy covers $50K. There are other coverages on things like work related vehicle accidents where if I am injured as the result of a crash in my work van which is also covered in my policy, I'll get payed $2,500 monthly while I can't be at work because of the injury. There is also an accidental death coverage if I should be killed on the job, the beneficiary gets payed X amount.
Here in PA, whether you have a garage or are mobile only the garage policy covers the vehicle's involved in any accidents so if you don't have a garage policy and run your mobile rig into a customer's house or car etc. you'll be paying out of pocket for the damages. It might be a good Idea to ask your agent about this in your state.
Paint burns and other incidental damages caused to a customer's vehicle as a result of any mistakes you make are not covered in general liability or garage policies. That cost comes out of your pocket so make sure to buy plenty of tape.
I am insured through "Erie Insurance" and it runs me $1,200 a year.