I have a 98 E320 with 82K miles. I have owned and maintained it since 60K miles. Other than fluid and filter changes, the only problems I had were the harmonic balancer failure and a sunroof which won't open fully.
I have read posts that window regulators are a common failure.
If you plan on keeping your car to 150K, some of the expensive maintenance items are:
Charcoal filter replacement (around 60K miles @ around $200).
Spark Plugs (100K miles or 4 years @ around $300).
Trans fluid and filter change (100K miles @ around $300).
The above costs are rough estimates, since I do my own maintenance I only know the parts pricing.
I plan to keep my E320 around for 300K miles and feel that at 150K miles the car is just starting to break in.
I feel that the cost of maintenance for the E320 is certainly worth it expecially when you consider the quaility of the ride you get.
FYI my 1988 Honda accord has been the cheapest car to maintain that I have owned - mainly due to the low cost of fluids/filters and parts.
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles