You will want to look at both General Liability (covers something like if a client were to trip over your buffer cord) as well as Garage Keepers (covers the car itself while it is in your care - even if you are mobile, the car would be "in your care" while you worked on it).
In regards to General Liability, most people start with quotes at $300,000 and go up to the millions. Before you select a coverage amount, you need to think about where you will be working. If you ever plan on working on a dealer's lot (if a client is having you detail a car prior to them taking delivery), or apartment or hotel parking lots, some of these type places typically require anyone working on their property to carry a certain dollar amount in General Liability coverage (typically $1,000,000).