You simply say "Mercedes 6 cylinder". If you're not worried about "period correctness", then I would have to suggest the 3.6 liter AMG engine used in the C36 (and E36 in Europe) in the middle to late 1990's. You also say "build a car", so I guess you're not limiting this to the W113 chassis?
Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both tops
1994 E420