if you have covered 800k kms in your car, congrats to you. not many would have such a chance to have taken their cars to such limits. talk maybe they can.
if you plan to drive it longer, just maintain it as normal, dont worry the wheels wont come off. just send it back to a good mechanic, in fact why not send straight to the merz approved dealer or service centre???? just a suggestion? have them go through your car. i dont think there are many that have covered that sort of mileage in a W211??
get them do a compression test and a major service like replacing all the basic wear and tear??? new hoses, mounts, leaky power steering rack, new radiator, new electric fan bla bla bla??? it will cost some money for sure, but hey, u have driven 800k kms. thats a mileage a normal person drives in say...32 years????