I did it again. Another Mercedes project.
First inspections revealed a broken vacuum hose leading to the brake booster (right at the check valve) that was held together with electrical tape. Not really a good sign that seemed to go against the service records in CarFax. I had no idea how severely these cars reacted to vacuum leaks until I drove it.
I was ready to give up, but ordered the $40 replacement line anyway. The car barely drove and if the line separated, the un-boosted brakes required a massive effort to get this giant car to stop.
There were a number of surprises I was very excited about. First - the reverse parking aids worked! Secondly, the automatic rear-view mirror worked! In all of my research, those two items were the least likely to work. I did strike out on the front left headlight wiper - it suffers from the typical failure for these cars.
So welcome to the family, S420! Hope you stick around a while!