For long distance cruising, a 500E/E500 would probably be a better choice. I also have a 98 C43 and the ride is very stiff. Your friend is obviously an enthusiast to some degree if he is even considering an AMG model so something fast and out of the ordinary, like a 500E, is probably what he wants. The 500E is faster, bigger, and more comfortable than the C36 or the C43. If you can't afford an E55, a 500E is the next best thing(some would say it is better). If I drove my C43 everyday I would eventually sell it and get a low mileage 500E or a late model E430. Once your colleague drives the cars he will be able to decide whether or not the C36 or C43 is for him.
98 C43 AMG
94 Range Rover