I can't speak for the gravel comments, but I think the MB flares make the ML look really nice as well as the the rear wing, if it does not affect your GPS antenna.
The dealer installed flares are similarly installed as the ones on the Sport package (the Sport package gets the AMG flares though). They're both separately mounted and glued on + clamped onto the fenders. However, there are gaps at the leading edges of the fenders, and this is where the debris that is thrown up from the tires can get into the flares.
I got these from Clair Imports already painted in black based on the VIN number. Also, these are MB Factory Flares and they do not have any gaps at all. If you take the time to put them on carefully, they will fit all the contours perfectly. I can't speak for any aftermarket ones. PS I don't have the sport package.
The Sport pkg ones are beefier. The flares for the non-Sport pkg MLs have not changed at all, other than the fact that they are now available pre-painted.