Four Questions About E300/e320

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I'm looking at trading my W123 300d for a newer E300 or 320, and have four questions that I'd very much appreciate some guidance on.
1) Is there a best year for these cars? I've looked at everything from a '90 E300 to a '98 E320.
2) Which years had the problem with bad wiring harnesses?
3) What kind of fuel consumption can you look for in normal highway driving?
4) Is the 320 twin cam engine with variable cam an improvement over the 300 SOHC. In other words, does the added complexity pay for itself?
Thanks for any comments.
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