Hmmmm, normally, I'd agree. I think I have a pretty good feel for what a car is 'worth' - and yes, E300TDs do appear to cost a lot of money for what they are.
On the other hand, you're getting one of the last properly built Mercedes with a new generation diesel engine - which is a highly desirable combination. Whatever premium you pay when you buy it, you're likely to get a lot of it back when you sell it.
They're very near the top of my list if I end up having to buy my own car again in the near future.