I'd check out that oil leak before making any decisions. Assuming your E420 has been well maintained it should have plenty of life left in it. They are nice cars.

At the Love Mercedes Tour earlier this year I drove just about all the mainstream models. The new W211s have a fresher interior and exterior, but drive about the same as the W210s (IMHO). The CDI was impressive. Oh, the new SLK350 is great too. Big improvment over the 1st generation.

Regarding depreciation, I would expect that an '05 C320 will depreciate much more in it's 1st year than an '97 E420 in it's 8th year. Based on postings and Consumer Reports, the new C-class has had a lot of reliability problems. I wonder if they have figured things out after a few years of production.

__________________
1998 E320 120K mi
1991 Miata 80K mi
2001 Ranger 109K mi