I would not consider both types a real issue. The the real threat to the W140 of the year you are interested is:

A) The A/C condenser issue

B) The Bio degradable wiring

C) All W140's with a 722.3-722.5 transmission will blow the reverse-gear lip-seal that will render reverse gear useless.

As a owner of two 140's, the two problems that you'll experience:

A) The vacuum pump that works the soft-close doors will weaken, and the pump will need to dissembled, and cleaned to restore the suction. Eventually the tail-tail sign is the pump will need to pull fuse No 9 to reset the pump. The pump mechanism CAN be cleaned instead of purchasing a new pump for over a $1k or rebuilt for $500

B) The vacuum diaphragms that operate the door locks will fail one by one, and the problem is very annoying, Yet even my 98 W140 has the problem. What happens is the plastic latches break holding the diaphragm together, and vacuum cannot be acheived. I fix them on my 96 W140 by holding them together with small cable-ties. It's been five years, and the cable tie trick worked!

If you have to follow ONLY one recommendation when you get your W140, replace the heater plastic tee before you do anything(or drive the car!!)-this will save the engine's life. When the tee breaks you have about 30seconds to stop the engine before you destroy it for good. Now, some W140's came with a metal one, but most are plastic.
If yours is plastic, buy one from the dealer only. Do not buy a aftermarket one...

Read this thread:

the evil that lurks under the hood... is really the devil himself - Benzworld.org - Mercedes-Benz Discussion Forum

Best of luck

Martin