Regarding the T-chain, gas and diesel engines are different. I will not be surprised if the 300SDL still has the original chain. I saw many 617 high mileage engines still had the original chain. I have not seen enough 60x engines to tell about their chains.
A good independent Mercedes shop is the way to go if you want to save money. I cannot comment on the $1200 price tag without knowing the details of the job. If you have to change every thing inside the timing chain housing, it will be expensive. It is a $8000+ engine, you just cannot afford to make mistakes when you perform the task.