The W140 ws available first with a 3.5 liter turbodiesel which, as everyone knows, turned out to be a complete reliability disaster. The car was called the 350SD and was available almost wordwide including the U.S.
Later on, the diesels were pull out of the U.S. market including the 350SD. Europe and the rest of the world saw the S300 TurboDiesel born into the market and became very popular, (mostly in Europe where gas prices are very high).
I ask the main question of this thread to the local M.B. service advisor once and he replied that the swap is NOT that easy. Many factors have to be taken into the account, including a whole new electrical/electronic system (which could raise your budget into the stratosphere, specially on a W140).
Although in some models the swap from a gas engine into a diesel one is not that tedious, most swaps from a diesel car into a gas version is almost impossible, to say the least.
CL 500, 1998
S 500 L, 1998
E 320 T, 1995