Hey guys I just bought an E550 2012 in Diamond White. I'm about to order a set of Forgestar F14s in Piano Black and wanted to confirm if the following size will fit. I did a bit of research on the forum but it seems most people have 20s and coilovers. I want to go 19s and no drop.

Front: 19x8.5, 245/35/19, ET35
Rear: 19x10, 275/30/19, ET42


Any help will be much appreciated!

You'll be fine as long as you don't lower the car..
But, I put 19" on my car without lowering and it didn't look good, so the springs had to be done. If you do the same, your front will be fine, but the rear might rub on the fender when going over the bumps with the car fully loaded. Do not go bigger than 275/30/19 with that offset if you're ever going to lower the car.
My 19" setup (car is now lowered on Eibachs):
Front: 19 x 8.5 - ET35 Tires: 245/35/19
Rear: 19 x 10 - ET55 Tires: 285/30/19
Here are photos, you be the judge. First one is before lowering.

There are multiple posts about wheel and tire applications: "search" is your friend.

The fitment looks good before and after the lowering.
I'm surprised the 10" wheels (what brand?) with ET 55 and 285 fit without rubbing, but like it.

Thanks for the reply guys. Would a 9.5 ET42 fit in the rear then?

Do 19x10. Offset 45. With 265 30 19 tires

The reason I want ET42 in the rear is because it needs to be ET42 to be deep concave. Would 9.5x19 ET42 work in the rear?

A 42 offset on a 9.5 wheel should be a nice fitment, I don't think that you will be disappointed with your choice.