I generally go with the feeling I get when driving the car for the first time.
For this 2008 e350 I test drove 2 cars prior. The first was not to my liking with a spotty service record and the second although a one owner with an impeccable service record was a real disappointment. The car I bought felt smooth and solid from the start although I have had to address some basic maintenance issues.
My wife loves her new older car and I enjoy that aspect as much as anything.
Having a reputable MB mechanic check out your car is an admirable thing to do on the objective side. The subjective side is another matter.
There is a book written by John Muir ( How To Tune Your Volkswagen ).
I used the service manual for my '66 bug. During the initial read you will find John's suggestion for checking a vehicle you intend to buy. He says that it is important to ask the question: " can I live with this car ". He even suggested that before you settled on your new ride it is advisable to climb up on the roof and assuming the lotus position ask the question again.
Subconsciously, I still do this and it has worked for me..