My guess is that the M104 engine was last used in 1997. Even the 1998 C280 went to the V6.
As for buying the car, I haven't heard anything negative about the longevity or durability of this engine.
One common problem with this engine would be oil leaking from the head gasket, which should've been addressed by 180,000 miles.
Another complaint of this engine is the dreaded wiring harness problem, but this only affected 93-95 models, and only (I believe) W124's, so this wouldn't affect this car.
A third complaint of this engine is check engine warning light issues, but they are usually related to low EGR flow, and the fix to this is quite easy and pointed out many times on this site.
All of the other issues regarding these W140's are related to the incredible cost of repairs, particularly with a/c and leaky evaporator cores. It seems the whole car was built around the evaporator, and it costs $3,000 in labor just to get to the part.
If this doesn't deter you (it wouldn't me), then on any car with this much mileage (mileage in itself doesn't bother me) I would need to have documented maintenance, either at the dealer or a Mercedes only shop with receipts stored in a file that I could go over.
Fwiw, I've been a passenger in privately owned taxi's in NY city that were W140's (always black) that had 400,000 - 500,000 miles that looked and felt brand new. I've driven in many Ford Crown Vic taxi's all over the U.S. with this much mileage, but man, did they feel it. You could feel every mile the cars have driven.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".