Actually it's the AC evaporator that can be a problem on W140s.
The cars are worlds apart. Try a few of each then come back with questions. If you like what the W140 has to offer, I doubt a W126 will suffice even for the substantial cost savings. If you have a fixed budget for this purchase, you're probably looking at the cheapest W140s which means you're not going to get a car with a good service history. A W140 in comparable condition to a particular W126 will cost at least twice as much as that W126.