K&N filters are widely reported on the various Mercedes forums as the cause of MAF/MAS oil contamination from the filter. It's good to hear that you have had positive experiences with K&N on your previous Merc's. Just to be on the safe side, my personal preference would be to use the AMG recommended product.

Non resistor copper core plugs (only) are widely recommended on the various Mercedes forums to avoid problems on the M104 engine. Again, it is good to hear of positive experiences with the iridium plugs. But still, it is my personal preference to use the AMG recommended product.

Non resistor copper core plugs are getting harder to source I agree.

Google K&N Mercedes MAF MAS and also M104 and copper core plugs to see more discussion of these subjects.

Also see Mod "whunter" thoughts:

https://www.peachparts.com/shopforum/general-information/197392-danger-resistor-spark-plugs-4.html

http://www.peachparts.com/shopforum/showthread.php?t=181047