I looked at 140's for a long time before buying. Nearly 3 years of research. My first choice was an early model 300SD until I found out about the issues with the 3.5L diesel engine. I then moved on to wanting an early model 300SE until I found out about the list of problems that the early cars suffer from. If you do the math the savings on the initial purchase are quickly eaten up by repair issues and then your still driving an older model car.

If your going to buy a w140 don't even bother with the early models. If an early model is all you can afford then you probably should not be looking at 140's to begin with as a late model 126 would probably suit your needs much better.

To answer your initial question there were a lot of issues with the early model evaporators that caused them to fail. Many of these issues were finally solved in some of the later model cars. The expense for the evaporator is not the part. Its the labor to dissasemble the many layers of the W140's interior. The 140 is probably one of the highest quality modern MB's made and this works against you when removing the many layers of dashboard and trim in order to get to the evaporator.

If your concerned about money any 140 does have the potential for expensive repairs. If you do your homework and learn about the car you can save a great deal of money on the repair side of the equation. If your the kind of person that looks at their car like a black box and likes to bring it to the dealer and say "fix it" your going to pay for that luxury and then some.

Some people might wonder "is it worth it" to own a 140. My answer is EVERY DAMN PENNY! My car is the most comfortable best driving car I have ever been in and I have driven many newer S class cars and a few Rolls Royce's and Bentleys. Its also the quietest car I have ever been in. The double paned glass and thick door seals do their job well for insulating the passenger compartment from noise. The Bose audio system with its 4 deck mounted subwoofers sounds great and the seats are really comfortable and provide wonderful support for long trips. On top of that I get 23.9mpg mixed city highway fuel economy in my car so its pretty economical to commute in even with having to put in premium fuel.