I'd think the running costs wouldn't be much worse than a modern S-Class. I'd go for the V8, the W engines are much harder to work on.
A friend had one and he loved it, but had to sell it when his commute got long. IIRC he was having to put $1200 in tires on it ever 35,000 miles. 17s are the smallest wheels that will fit too
I was really nice taking that car for a spin. I'd love to have one someday. It's too bad they didn't sell the TDI here
Also, unlike with a Mercedes, you can get a proper scan tool (VCDS) to work on them for cheap. So it might be full of computers, but you can at least diagnose and repair them yourself.
-J