Yeah, 1998 and newer Mercedes with six cylinder engines are V6's, not inline sixes. The factory fill is also synthetic oil, not conventional oil. May owners have switched to synthetic oil in 97 and older cars, but some (like me) are afraid of oil leaks. The switch to synthetics and oil leaking, as well as the merits on the inline six vs. the V6 have been debated at considerable length many different times.
There is nothing wrong with the M104 engine. It will last much longer than you will probably keep the car. Many things will go wrong before you have a catastrophic engine failure or need to rebuild the top end of the engine due to valve guide / seal wear and head gasket replacement. The techs say the bottom end of these engines virtually never wear out.
I don't know if you are out of the woods with the evaperator. I know from many owners' experience on this site and from comments from my local tech, that mass air sensors are still a continuous problem, even on newer models. You're only talking about a $300 part and 20 minutes of labor. It might be the easiest thing you ever replace on your car.
The oil leaking from the head gasket seems to never have been fixed. You don't have to replace the gasket. It will just leak oil.
If you've read enough, you may have seen that the many problems these W140's have were common on the earlier years, and seem to have been largely worked out. I don't know what year that happened, but surely by 97 you'd think you'd be in the clear, wouldn't you?
But the point remains, I'd get the warranty. I don't think you'll find many members on this site not recommending a warranty on a W140. That's not saying the W140's are not great cars. Polls on this site suggest that more members think the W140's are better than the newer W220's. The W140's probably represent the pinnacle in engineering at all costs by Mercedes. They're just expensive to own.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".