You have the same year and engine as mine, so maybe I can shed some light on this:
Need a bit more info on the overheating problem you describe...are you losing coolant (i.e. boiling over), or are your engine temps higher than normal?
It gets pretty hot in Texas, and I am OK with engine temps getting near 105C according to the gauge. This is in excruciatingly slow stop-and-go traffic. The aux fans will kick in at about 107C, and lower temps down to about 100C. BTW, this is with the A/C going at full blast.
In cooler days and highway driving, temps stay at around 85 - 87C.
Without getting into the expensive cooling components (which will fail on their own right), I would check two other things...the aux fan sensor, and the expansion tank cap.
The aux fan sensor is located on top of the water pump housing. It is the two-prong connector. If you remove the connection while the engine is running, your aux funs should start going at full speed. They are designed to operate when the A/C is on, since the additional radiator used for the A/C will now be expelling heat in front of the engine radiator. About $30 + shipping at FastLane.
The radiator cap on the expansion tank is desinged to hold pressure at 20 psi or 140 (bar), which is what the cap should say. If it's not doing its job, it can't hold pressure, and thus the boiling point is lowered, so it overheats faster. This is about a $10 part.
Coolant leaves a white alkaline residue, so leaks should be fairly easy to spot. Check for those.
Problem #2 is not related to #1. My first guess would be the A/C idle sensor. Not sure where it's located, but it's job is to adjust the engine idle speed when the A/C is switched on. Other guess would be the OVP. The OVP causes other problems, but when I first had problems, it would be stalling when A/C is on.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car