If your GL450 is due for its first thermostat replacement (in my case 107 k miles), and the following parts have not been replaced, replace them together to save on the labor.
belt drive & cooling system:
thermostat
coolant reservoir
upper coolant hose
idler 1 & 2
tensioner
belt

the following items could be or will soon be leaking:
engine oil filter housing seal
engine oil cooler seal
steering fluid reservoir & o ring

cam position sensor (4 of them) due to oil leaking around them.

I learned this in the hard way. take me four trips to replace them. and each time have to remove the cover, radiator fan box, and the thermostat has to be removed and installed 3 times.
should have replaced all of them in one operation if I knew this earlier.

 

I would group them like this, assuming you do not care to do them all at once:

thermostat
coolant reservoir
upper coolant hose

idler 1 & 2
tensioner
belt
steering fluid reservoir & o ring
cam position sensor (4 of them) due to oil leaking around them. (kinda surprised to hear these are all leaking; you know the sensor, which is small, and the adjuster magnet, which is big, are two different things)

engine oil filter housing seal (independent of other work unless I am missing something)
engine oil cooler seal (you have an oil cooler?)

 

oil cooler seal (left), housing seal (right)

 

On the V8s at least, you have to remove the thermostat to get the oil cooler assembly off. Ive done the oil cooler and you can't get to the bolts to renove it whilst the thermostat is in place and you also need to remove at least one tensioner.

 

Definitely very good advice get all that stuff done and out of the way so you save a lot of time and especially money on labor I have done the same I’m glad I did

 

Lots of them are all interlocked.
But i really enjoy working on the GL450. easy to work on, oil change (no need to work under the SUV), spark plug change, etc.
well designed.