Lately, it seems as though your old 300D may have almost as much value as an E320. The prices I see on ebay, wow!
The wiring harness problem is generally thought to affect 92-95 model years. I have heard of a few '96's being affected as well. '93 to '95 I believe are ALL affected guaranteed.
On the highway, you can expect around 25 mpg.
The '97 and '98 6-cyl E's use a v-6. Your 4th question is almost religious in nature
I have both and think they are both good. The E320 M104 (twin cam straight six) does make more horsepower than the M103 and has about the same fuel consumption.
Good Luck,
-GH