I know this has been touched on many times but can you tell me if these wheels would fit?
Front 9 x 18, et 42.
Rear 9 x 20, et 40..
What would be the best size of tyre?
The car will have been lowered 30mm and I am hoping to have slk55 6pot front calipers on. The car is n/a.

 

No...not really. 9" on the front is asking a lot. Going from 7.5et35 to 9et42 will encroach on the strut and you'll likely rub the inner fender liner, especially if your lowered. You'll also need to verify brake clearance of this wheel if you're using bigger brakes.
You'll need a 20mm spacer on the back to get that 40 offset back down to the stock 20.