However, perhaps I could throw another option into the mix - have you considered an Audi A8?
I could second this recommendation. I owned an 04 4.2 V8 and found it to be a thoroughly pleasant experience. Like Simon mentioned, I bought mine preowned and lost very little when I sold it as the bulk of the depreciation had occurred in the first 12 months. If you are considering a BMW 7 or a Merc S, I really believe you should at least test drive an A8
