I have done hundreds of chain failures over the lats 27 years going back to 450's.
In all V8s, from 71 till the 4 valve came out in 90, there have been two timing chains. One for the 3.5, 3.8, 4.2 motors and another for the 4.5, 5.0, 5.6 motors. Wide and narrow blocks I call them - did you know that one can instantly tell a wide block from a narrow block by looking at the airfilter housing mount to the valve cover. The narrow engine has a extention bracket to get it the width of the airclleaner. The wide blocks bolt the aircleaner straight to the mount to the valve cover. The brackets take up for the small motor being about an inch narrower at the valve covers.
The only reason I can think of to replace the chain is due to stretch. After about 150k there is about 10deg retard in the right (following) cam. Twenty-five years ago i used to dial indicate and repair this timing issue with offset keys. Back when there were aluminum rails I don't think we saw the failures.
I do not think there is a chance of a chain failing due to wear in the first 250k. All the failures are due to the broken rails. In the post where I advised to just replace the rails the car only had about 100k, if I recall. I made the comment the way I did to place emphasis on where the risk lies. We almost never just due rails but when we do it someone is paying a lot of labor, it makes no sense to do part of the job.
As to the price of this car. The selling price is to be compared with reasonable facsimiles. The sum total of all such risks as you are questioning are the reason the car is being sold for a lot less than it did new. To try and use statistical projected failures as a line item negotiation misunderstands the used car concept. Why not use a list of all systems and their statistical percent of new usefullness and asign a value for the total (based upon the percent of new price). Obviously this would leave you in a worse position as I imagine there is 75% usefullness still in a nice 126 body with 120k. Should be worth somewhere over 20k...... ahhhh but the market place makes price.
Bosch Master, ASE Master, L1
33 years MB technician