Hi, and welcome to Mercedesshop!

You can have a look at the thread below that was posted earlier, today. It will answer many of your questions

87+ MB diesels

As for mileage on a Mercedes,
1. Buy the car with the fewest miles possible (in some cases, though a car that is rarely driven is like an airplane never flown - it's not airworthy)
2. Try to find a car with as complete a service history as possible.
3. No matter the mileage, if all things mechanical check-out, adhere to a religious routine of preventative and routine maintenance. 150,000 miles is just getting broken-in. 300,000 is getting up there but still, a car with lots of life left given attention to preventative and routine maintenence.

Any questions, don't hesitate to ask. You'll be glad you've made an informed decision.

Best of luck.