The general consensus is that 1998 and later W140's are "safe" to buy as all the bugs and quality issues were worked out by then.
I don't know if the evaporator problem was ever completely fixed, but at least by 1998 the failure rate was much lower.
If the price was right and the condition was good, and the car had a proper maintenance history, I wouldn't be afraid to buy it.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".