I have 9 x 18 35 ET AMG monoblocks on the rear of another SL. There is maybe 1/2" left to move the wheel to the inside before hitting one of the suspension arms. ( Pretty sure it is the froward toe link ) Also the inner fender / frame section limits movement.
I think some 98 and up cars have 9 1/2" on the rear so 10" is likely the upper limit without cutting metal.

There isn't much room to move towards the fender.

The least intrusive swap would be a C class rear sub with a 198 MM diff as the SL used this past 95 ish.

As for wheels the plan is to widen steel R class 18" spares, this will allow me to run any offset.