Only one reason to buy from a Mercedes dealer, warranty and you want lots of it. So buy one with low miles and lots of warranty if you expect something cheap to keep, otherwise get a cheaper one on the market and be picky with inspections/history etc and find a nice clean one with some miles for lots less the dealer wants for one. I spent 8 months looking for a R320, looked at dozens (lots of driving to large cities to find em) and as soon as I saw the one we bought I knew, no dents, no scratches, clean light grey carpets, no salt exposure etc. Had it 2.5 years now no issues to date, just turned 100k miles. They are tough on tires (funny wear patterns/choppy) but otherwise great, kids love the dvds and space/comfort. Almost too quiet nothing to lull them asleep!
It is a completely different world out there from the w123, I have some I know the differences, but the modern stuff is throw away, replace parts dont try to fix it right kind of society. Lots of power and fuel economy (better then a 240D) but when something goes wrong it will be very hurtful. I see the ML and R well under 20k around here now (not lots of them but enough), keep looking.