I've fitted 10" rear wheels to my E500 and found that you have a great deal of clearance for outside extension (up to 41mm) before you will experience interference, even with a lowered suspension.
OTOH, there seems to be very little inside clearance in the rear suspension, which means you have to be very careful about the range of offsets you consider.
Your 18x10 ET30 rims will provide 20mm less clearance than stock on the inside of the wheel. I've fitted 10" rims without problems that had offsets of ET19 to ET22, which provides 7 to 9mm less inside clearance.
The most I've seen from a wheel manufacturer's recommended 500E/E500 10" wheel was 13mm less clearance on the inside. 20mm seems to be beyond the acceptable range.
FWIW, I don't think your wheel deal will work!
