Welcome to the forum DwiRic. You have asked a really difficult question but I'll share with you my thoughts on the subject if I understand your question correctly.
Firstly, with the number of miles travelled by your car I would expect it should easily travel the same distance again assuming the engine has been correctly cared for over its life to date. 500,000+ miles is not unheard of from the older MB engines, especially the diesel. Given the age of the car, this would suggest another 30 years of service from the engine so in this regard it would easily be worth looking after the body and interior.
Now, it is difficult for me to say that the W116 cars that we all love so much are
just a 4 door sedan. It would be easy to spend large sums of money restoring a W116 but you should ask yourself whether you expect to ever get your money back. They are not a coupe and are not a convertible, either of which would generally have a higher value placed on them by their very nature.
Many of us would be willing and do restore these cars. Why do people spend the money? As a show car, I would not expect them to compete with more special and unique cars but given the life expectancy, a respray and re-doing the interior can prove to be worthwhile if you will use the car. You should not plan to be able to sell your car and get your money back that way.
As I am sure you are well aware, these cars are great to drive and very reliable. There aren't too many 4 door sedans with the same style, class, handling and comfort as the W116 and this may be enough of a reason for you to spend the money.
I hope I have given you something to think about. Personally, I plan to completely restore my W116 one day to 'just left the show room' condition. I just need to save some money first
