I am a sales rep at a large NE MB dealer. To the best of my knowledge (which is usually good, but no one is infallible:
1) The R class cannot be fitted with a hitch. I have checked with the parts guys, and have lost a sale as a result. There is curently no MB part number for a hitch on the R class. Maybe in the future, but not now/. You can possibly go after-market, but since this is a unibody (no frame) I would be leary of how much you can tow.
2) The ML (any model) can be fitted with a hitch, either at the dealer or in production. 5000 pound tow capacity. New ML diesel has 398 foot-pounds of torque, and a wonderful purring sound. Very cool.
3) The GL 450 (very cool car, well worth the money, see CAR AND DRIVER, Oct 2006), can be fitted, 7500 pound capacity.
4) If you are in mid-north NJ, I would welcome a call-email from you.
-Paul T