My S500 is just over 120k miles. Does anyone know what's required to change in addition to the typical A or B service?

 

It should be in your service manual. The spark plug change is at 100K miles, if this has been done then already, then you might need a radiator flush, fuel filter, cabin filter and engine filter change. They also test for contamination in your brake fluid and might recommend a replacement(I believe every two years).