So I just bought one of these - the pre-facelift w221 though which was a conscious choice for many reasons. You can find my thread here:
New project - what to do? Ideas please!
It took me a fair while to find a good one - so take your time as there are a lot of ex-corporate or ex-hire dogs out there. Pick a good one and they are brilliant, but still very expensive to maintain - and if something goes wrong then you'll be paying ££££££ especially the bluetec'd varients (hence why I bought a pre-facelift). Now I restore cars and if you have the kit, source proper OEM parts (not just aftermarket parts) and do the work yourself then things become far far cheaper.
Looking at the last owners invoices he spent £1200 last year on a service, a brake shoe change and an automatic oil change. Previous years seem to run higher, but they were done completely by dealers not merc indies and dealers.
As a comparison, I've just done the front brakes myself, which (because of a stuck piston) I doubt if I'd have seen change from £1000 if I'd had a dealer do it - as it was it cost me £170 and a day of my time. It'll cost me another £20 and a few hours work to get fully restored which I aim to do next week along with painting the calipers.
Hopefully that gives you an idea, but remember my car is a very well maintained example - get a poorly maintained one and you'll be into more cash. However, short of another £300 on tyres I expect that to be mostly it for the next year on repairs (unless the intermittent minor radar sensor fault turns out not to be a connection in which case I'll spend another £200 ish on a second hand part, not really necessary but I'm a stickler for perfectly performing cars!).
Anyway, what I'm saying is that as a comparison to what you already own, expect to pay out serious money if your not doing the work yourself.
Ok, that's the bad - the good is that this is without doubt (and except for my short stint with the Phantom) the nicest ride I have had. They're lovely so go for it if you can afford it.
If you're not doing city centres go with a diesel for economy - they'll do 200k miles before having serious issues. Go with a car that has all the options/toys. In general cars usually have most of their manufacturing defects 'outed' and sorted in the first 50 to 70K miles, so I tend to buy after that. Personally I wouldn't touch a bluetec, but many have and they are not as bad as they are made out to be.
Anyhoo - hope that helps,
QX