x2 what Brian said.
I am in exactly the situation you have in terms of a very limited local market. When shopping for my first car, over months I wasted time, money and nerves driving here and there to see cars. Total loss. In the end I found one 15 minutes from home.
Based on that experience, dealers like those are my last stop when shopping. To start with, in my experience most of them tend to provide creative versions of reality in order to get you onto the lot. If and when you arrive, you find the car is far substandard to their talk. And they know you've travelled to see the car, which suggests you are hot to trot for the car, which kind of kills the "I'll walk away if I don't get a good price" option.
As a general comment, buying your first diesel Mercedes at a distance wouldn't be the easiest plan. Much of any savings is going to be eaten into by logistics: travel to go see it, hassles of finding and then scheduling and paying for a reliable independent shop to inspect it, time needed to drive it home, etc. Could involve at least two trips back and forth. If you can swing all that, great, just something to bear in mind, especially the need for a trustworthy and qualified shop to look over the car as a condition of your purchase offer.
If you are in the mood to travel, however, did you see this?
Two wagons for sale