公正行估價 won't be this high, they won't do it according to his request (send to benz). Have you got his business card? Have you found out where does he work? (you may have a clue if he's ordinary working class or someone who has money to waste on solicitor fees)
You try negotiate with him.... if you two not settle on your own, you will go thru insurance. The official procedure is insurer would get 行估價 to obtain quotes. From my experience, 公正行 would not use official quote from benz (since Benz doesn't repair but replace it, and insurance usually won't agree with this as far as I know). So what ends up would be only far less than what benz ask for. If he's not satisfied, he has the right to lodge a claim against you through civil law. Then both of you would waste so much on solicitor fees.
So what it means, if he insists on benz repair shop, you say you don't havet money, you can only claim insurance, but insurer may not agree to send to benz for repair (it's not upto him/you). If he's not satisfied with what your insurer is willing to compensate him, he can sue you through civil law, but he has to bear the fees as well.
Talk to him nicely but with firm tone. I guess he knows you are P licensee, and foreigner likes to play with official way. You better cross check with friends / relatives who work for car insurance, they know how it works and give you insight.