Still hunting for a decent (inexpensive) diesel…

In the 240D & 300D cars, how long do the 4 speed manual transmissions usually last?

I am asking because I am going to go look at a 1979 car that supposedly has only 127,000 miles on it, yet the owner says that the transmission is making ominous grinding noises (to the point where he says it shouldn’t be driven back if I decide to purchase it.) If these trannys usually last much much longer, then I am suspicious of the relatively low mileage on the odometer. (I know that the way a car is driven will make a huge difference, but I’m looking for a general idea as to how long they last.)

Thanks again everyone!