W140 Diesel

Hi
About a year ago I drove a 94 S350D.It had 320000 km and was running better than any diesel I tried.So much smoother than my 300sdl.
I hear these cars(or engines) have a bunch of problems so my questions are:1.Does anyone know for sure that the 95 model is OK?
2. If the engine was replaced on a 93 or 94 is it safe to buy one ?

What are the changes ?

As far as evaporator goes if it has been changed will it still be a problem or they have a better design ?

Thanks for the info
John
90 V8 q for now