I'm looking to get some new rims for my 2013 E350 Coupe with the USDM AMG sport trim. Would
18x8J ET35 front wheels, and 18x9J ET38 wheels be a good fit?
It seems like the front is +10mm further out and 11mm out in the rear. Will this be an issue for clearance in the rear, if I get half inch wider rims?

I studied half a year wheel sizes for mine (E500 AMG Package). Yours will fit fine. Rear OEM wheels are pretty hidden in the arch, and undersized in diameter, so feel free to „add” them 10-15mm (comparasing OEM) to the outside of the body.

Undersized? Wdym by undersized? Do they look proportioned better with 19s instead of 18s?

I meant, that OEM wheels ET is pretty high, and additionally OEM diameter is ~640 mm. High ET, small wheel diameter and stylistically quite massive back end of the car are causing impression, that OEM wheels are undersized.
lower ET, little bigger diameter (with rim, with tyrewall, whatever) and look gains