Mercedes ML350 Bluetec - How big a TT can I tow?

You shouldn't have an issue. The weight police may be on you, but that is the norm for anyone who tows small. My Jeep Liberty CRD with ab engine tune and transmission tune tows great for its size, so I think you should be happy. Just remember, it isn't a one tone dually and you will feel the trailer behind you.

Also, if you don't have a hitch, depending on year, this one is rated at 600 TW.

http://www.etrailer.com/Trailer-Hitch/Mercedes-Benz/M-Class/2012/C13117.html?vehicleid=201211845