First, your oil cooler and radiator are two different systems. For your coolant change, there should be a engine block drain plug somewhere on the side of the block. Ususally it's on the passenger side of the engine, and is 19mm. You open that and the drainplug at the bottom of the radiator. Open the radiator cap and pump clear water through it with a garden hose until it runs clear from both drainplugs. For the oil in the oil cooler. Some older models do have a seperate drainplug on the cooler itself. If yours doesn't have one, then there is no practical way of getting the oil out. Some have suggested disconnecting the lower oil line, but others say you run the risk of breaking the fitting. Oil coolers are not cheap!!!!!! I would just stick with changing the oil regularly.
For the thermostat, there should be about 3 bolts that secure the thermostat cover down. I am not familar with your car's engine, so I can't say exactly where it's located. Usually it's in the front of the engine, at the top. The upper radiator hose will connect to it. Again, I am guessing with your engine. One thing is for sure, be careful when removing the bolts. The bolts are steel and the housing is aluminum, this makes for galvanic corrosion. I once broke the head of a bolt due to it being seized up. I would soak the bolts with Liquid Wrench for a long while, then tap the bolt heads with a hammer or something similar. The gently undo them.
As for the temp. that the thermostat opens. I believe it would be 80C.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section