W124 shopping: what to look for?

[BenzV8]

Quick background: my venerable old Honda has finally given up the ghost. This presents a convenient problem: I don't want to subject my immaculate C63 to the punishment of daily commuting. So, why not try to get another Mercedes for commute duty without breaking the bank (again)? I think this is how the addiction begins, isn't it?

I've settled on a W124, and found this lovely black/saddle E320W with relatively low miles on eBay. Before I begin spending time and money to perform due diligence on this car, I have four questions for any W124 aficionados out there:

1. Based on what I know about the W124, it seems to be what I expect from a Benz: reasonably sturdy and reliable if properly maintained. I'm aware of two common issues (head gasket, engine upper wiring harness). Other than these problems, are there other recurring issues that require attention with these older cars?
   
2. The valve cover seems to show some discoloration/yellowing. Is this a cause for concern? Suggestive of high temperatures or something like that?
   
3. What are those squares affixed to the interior trim panels of the front doors? Some kind of knee bolsters? Are they removable? Never seen those before in a W124 (admittedly, I'm far from a W124 expert).
   
4. Price: I would expect this car to sell at the high end of market pricing, but $15k is nearly 3x NADA retail book value, which seems absurd. Am I missing something? Would you pay anywhere close to this asking price?

Thanks for any info you can provide.