I've heard of 104 motor head gasket failures as early as 20k miles. I once had a 94 E320 in which the original failed at 35k and the replacement failed at 67k. They have improved the gaskets but it is nevertheless a flawed design. The block is cast iron and the head is alloy and they expand and contract at different rates putting pressure on the gasket. It is not an uncommon combination but it is rare in such a long straight six. About all you can do is put some research into finding the best gasket and make sure you've got a good mechanic. You can find out more about the gasket suppliers with a search above. There is a nice article on the subject by Steve Brotherton - do a google search for "Steve Brotherton Mercedes head gasket" and you should find it.