The SDL should be happy with the 15's thats the only wheel MB offered them with.
t walgamuth do yourself a favor and don't drive a W140 with 18in AMG's for example. The amazing ride will make a die hard W126 fan cry with envy.(like me) No matter what I replace on my suspension(which is everything, at great expense I might add), even if I use all OE parts(which I do) their is no way I can achieve such a smooth road imperfection absorbing ride. Oh how I wish I could glide over pot holes not noticing their existance. Road imperfections that shake the W126, you fail to notice in a W140...Corners that make my Bilstein sprung W126 roll, just don't affect its younger brother. One only needs to compare the front ends side by side to see why.
When I drive an S I don't want to be bounced around, if I wanted to feel like I'm one with the road I'll buy an M3, thank you.
All I can say is the W126 is lucky it is so fine in other areas, and looks so darn good.
So in conclusion slap whatever wheel looks best and fits best onto your W126, the ride is going to be marginal no matter what you do.