I know the Yakima Racks Lowrider car rack system is engineered to mount to your 2000 Mercedes ml320 roof rack siderails perfectly. The Yakima Lowrider system includes 4 Lowrider towers that mount to the siderails and a pair of heavy duty round crossbars that connect to the 4 Lowrider towers. This system provides the platform foundation you are seeking. You cna then connect any of Yakima Racks gear carriers right to those crossbars.
You might want to also check out
these various Mercedes roof rack system pictures or
these Mercedes Benz car rack pictures as further resources.