IMO collector cars are usually cars that someone seriously dreamed about owning at an earlier age and then could afford it at a later date. Many offer special features including a special engine (performance), a special paint scheme/interior and limited production. Some were also upscale cars that most couldn't afford when new like a Ferrari. Other cars become collectors because someone may have owned one years ago and it brings back great memories (many muscle cars) and they want to bring back that era in their life.
So now you need to consider how many people truly loved the W140 sedans and or coupes when they were available for sale as new and or how many previous owners would want one again. Were people commenting "I wish I could have that car" when they saw one? Honestly I don't know, I wasn't a huge fan but thats just me.
The thought of maintaining one might make sense since I doubt very few will exist in 10 years but I'd relegate this car to weekend joy ride duty to decrease maintenance costs and help maintain original paint/interior.
There are many cars in MB's history which have excessive maintenance requirements including the 600 and the 6.9 with air suspension, both are collectors. There are also many that offered very little maintenance requirements, none of which are collectors.
IMHO the W126 560SEC is already a collectors car, it's rare, many wanted one and they're beautiful. But again that's just my opinion.
