Popcorn? Now you may need it! May be even a sick sack...
Mini and Airstream Argosy.swf - YouTube
ML is a good example of irrelevance of towing capacity and a proof that TC, while may reflect technical capability, serve other goals as well, marketing first of all, can vary from market to market and can be inflated or lowered for no mechanical reason. ML's TC in Europe and most other places is 7715lb regardless of the engine and was well validated by thousands of people towing various things and this is the only number I would respect, at least to some extent. When ML II generation went on sale in US in 2006 it was rated at 5000lb. In a year or two a 2WD model came and...4WD model's TC jumped to 7200lb, while 2WD was rated at 4600lb. No major mechanical changes were done to the vehicle during entire production run. Same powertrain, cooling, brakes, springs, GVWR, same lousy hitch receiver which was improved to acceptable level only in 2011 and so on.
My vehicle is 2WD and I spent considerable amount of time researching it in details and getting separate professional opinions which were unanimous: The only difference of 2WD from 4WD is just...the fact that this is a 2WD! And these 2 versions are equally capable, as long as I am not trying to pull 7700lb boat on a slippery ramp.
The Passport I am pulling is approx. 4900lb empty/540 hitch, 7200lb gross, 5800lb in reality. I have a weight receipt with 3 people inside from 1 year ago; there was somewhat less stuff than now + fuel tank was only 1/3 full so 100lb should be added to the rear axle. WD was engaged.
ML63AMG on your picture is a great machine, but quite pricy. It can certainly accelerate the trailer up to triple digit speeds but its unsafe anyway and nobody would like gas mileage. Basic V6 is simply enough to do the job of pulling TT at 60-65 mph in almost all real life situations