I would not buy a Mercedes without an extended warranty, if I ever buy one again.
I have read on this forum warranties paying upwards of $20,000 on the S-Class for repairs!!
Oil change is going to be the least of your concerns
If you go to the dealership for every repair, just the MAS will cost you somehere around $600. And it will fail
I have spent about $10,000 in repairs, including wear items on my 94 E320 which was a 1 owner vehicle with low miles when I bought it. I take care of my cars more than the average consumer, so the expense would have been less, if I had neglected some items, and avoided the dealership a little bit more.