We all say to check every single electronic option and that is excellent advice, but the good news is that you have been assured the vehicle has a FULL Mercedes-Benz service history.
As long as the car has been kept within the main dealer umbrella then the car should be pretty solid. Because it is an early type vehicle then when it had the early service work carried it, it would have gone through a strict quality control check.
I have not heard of any rust problems on this model but check the black trim and the wheels for signs of electrolysis.
The E55AMG is a beautiful car and one that most folks can only dream of owning. Not cheap to run, but what price do we pay for that major grin factor.
I just disliike black or silver coloured cars just because they are sooo common