ET & spacers
For example IF the aftermarket wheels you want to purchase have offset of 35mm (as in this case) and your ML320 original wheels have an offset of 52mm you will need hubcentric spacers of at least 12mm to make it fit as stock.
I beleive you have this backwards, correct me anyone if I'm wrong, but here is my understanding of Offset and when spacers can help.
1 - Offsets with high positive numbers push the centerline of the rim width closer to the suspension, 54 is a high positive offset.
2 - Offsets with a lower positive number, (or negative number), push the centerline of the rim width closer to the outer fender, 35 is a lower positive offset.
So if your car is designed to fit wheels with a 54 ET rim, and you purchase et 35 rims, and mount them, they may possibly rub the outer fender, or in extreme cases actually stick outside the wheel well.
With this example, adding any spacers will only make the wheels stick out farther, the lower the positive number the more they move outwards. ET 35 and 12mm spacers = new et of 27
Spacers would work if you purchased rims with an et of say 66, then a 12mm spacer moves the rim outwards and effectivly gives you a new et of 54.
So spacers only work in one case, when the new rims are too far inside the wheel well. (You don't add the spacer width to the wheel et, you subtrack it from the wheel et)
A real life example is my CE, it takes et 40ish wheels, wheels in the low 30's, 20's and teens rub the outer fender or actually stick out of the wheel well.
Monoblock Wheels from a W126 are often et in the teens, and are usable on only a few other cars, they will never fit a car designed for et 40, 50 or 60, they will stick out like tires do on some of the jeeps and trucks you see running around.
On the other hand I could take one of your et 54 wheels, put a 12mm spacer on and effectively have a new et of 42, which would fit fine as long as they aren't too wide.
The width of the rims also add another dimention, as you widen the rims, in general the et required is now a lower positive number.
Finally, the use of spacers gets mixed reactions, some swear by them, others think they are unsafe, and if you do get wheels that require spacers, you will also need longer stud bolts.