I am an engineer at Freightliner - here is what I know that some might find interesting . .
Mercedes 4000 (12. 8L) is assembled in Brazil
Mercedes 900 (4cyl = 4. 3 or 4. 8L, 6cyl = 6. 4 or 7. 2L) is assembled in Germany
Both engines are serviced through Detroit Diesel and Freightliner/Sterling/WesternStar
Big Cummins engines are not available from Freightliner, ISC and ISL are. this is since the oct 2002 emmissions change. Due to the emmissions requirements changing in 2002,4,7, and 10, most OEM's have had to drop at least one engine supplier in order to have enough engineering resources to keep up with all the changes that are happening in a short period of time. Freightliner dropped cummins, Volvo dropped CAT, etc.
Keep in mind - at one time Freightliner offered the following in heavy engines - MB4000, DDC60, CAT C10,12 and 15 (formerly 3406), Cummins ISM, N14, and ISX. When ALL of these have major changes at the same time (dictated by the EPA) - its a huge amount of changes. Other OEM's are in the same boat and have had to limit choices. Daimler owns DDC and Freightliner, so it was down to either CAT or Cummins - I am way too low on the totem pole to know the details of why CAT stayed.
Mathew