I don't know about the rest of you guys, but I have never changed a water pump because of anything but a mechanical seal leak. To correctly diagnose your problem a little more data is needed, like, temperatures you reach at idle, temperature response to driving at highway speeds after the idle high temperature, A/C on and off for that cycle, and so forth. It also helps to turn the heat on full and see if that has an effect on engine temperature.

Cooling system performance is affected by nearly every feature of the system, so it is possible you have a worn out water pump. Before I assumed that I would boil the thermostat and check its response vs. temperature of the water, the condition of the A/C condenser and the radiator, as well as the space between them, and I would check the main fan drive. At some point MB started using viscous clutches activated by the air temperature coming over the hub (after going through the radiator). These things eventually lose the oil. There are threads on how to get a Toyota supplied dose to refill and how to do it, if that turns out to be the problem.

Good luck, and I hope this helps. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)