Nice car but wrong engine. Most will want either the diesel or the V5. I like 6 pot engines, always have but reckon in a car that size, you want a bigger engine than that, or a turbo diesel to give it some mid range shove.
Bear in mind that in UK S320CDI accounts for a thumping 80% plus of s class sales.The S Class market doesn't fit the norm in some respects. However good you think the diesel is, it just doesn't suit the car as well as a petrol engine. If I was to get another S I would get the 500. Another factor is that some people simply will not buy a diesel model (Arabs for example), so for the right buyer, the price won't suffer as much as you think. There are a few 350 Vianos tooling around London - another case in point.