it is difficult to diagnose a problem over the internet but if I were guessing it sounds like a transmission issue because when you placed your foot on the accelerator the car barely moved. Perhaps the transmission lines were either crimped or punctured which affected the transmission. The clunking sound happened before you hit the curb which means something was going on prior. I recently had a transmission problem and had to make about fifty phone calls before I settled on a shop that I believed knew what they were doing in regards to Mercedes Benz. Good thing because my transmission case was cracked and they replaced it. Had I picked another shop, I'm not sure I would have received that level of service.

Most tranny shops are focused on domestic cars and trucks, so be sure and find one that works on Mercedes Benz ; also pull a Better Business Bureau report (A rating or better), get references from other transmission shops, etc. Also, ask the shop about their warranty policies.

Or, take it to the dealer where you KNOW you will receive certified Mercedes Benz service, at a price of course.