I've thought a bit more, and have some more questions.

I'm not looking to have the car for a daily driver - I work from home, so the only commuting I do is in my fluffy pink bunny slippers. I'm expecting to do significantly less miles in a year than average. The miles I do put on it will be a mixture of short trips around town and long highway cruises (at least 100 mile round trips on the interstate).

Seeing as how these cars are both in the dreaded biodegradable wiring harness years, should I expect to replace the main wiring harness on either car as soon as I get them if they've not already been done?

I really need to start changing my own brake pads and oil - are there many differences between my 300E2.8 and either of these two for small jobs like that?

I've changed out parts such as the fan, fan clutch, bearing and blower motor on my 300E, and believe that I could do bigger jobs like water pumps, belt tensioner and alternators- how are either of these cars to work on for jobs like that?

For the bigger jobs like head gaskets, timing chains and so on, which car would be most economical for me to take to the dealer to do?

Update:

I keep on forgetting questions: Would the 300SD be a possible candidate for conversion to SVO/WVO? How does the engine run on biodiesel? Has this been done before (I can only find references to the W126 300SD's, not W140), and is there any evidence for a reduction in the engine's lifespan?