My running costs are pretty similar to AlexM's too. Just totted it up and I've driven just over 56k miles in a Biturbo E63 since April 2014 and every one of those miles has been enjoyable.
I run mine on Momentum 99 pretty much exclusively, partly because I have to detour off my usual route to get V-Power, and partly because I object to Shell's premium pricing (ludicrous, admittedly, in the whole scheme of E63 ownership). Even when I've run successive tanks of V-Power my bum-dyno has detected no difference in performance to running Momentum, and mpg is no different either. So I save around £8 a fill and amass ClubCard points at a pleasing rate that gives me a couple of free Eurotunnel crossings a year
For tyres, I've run Pirelli P-Zeros, Conti 5P's and Goodyear Asymmetric 3's and of the three I reckon the Goodyear's give the best overall performance, with the Conti's a close second and, if you take cold & wet weather conditions into account, the Pirelli's are a distant third.
The M157 engine does use a bit of oil, but as others have said it gives you a warning in the instrument cluster if it needs topping up. Be aware that the warning will often only appear during sustained motorway-speed runs, so it's worth checking the dipstick every 3-4k miles too.
An MB ServiceCare Plan (£46 / mth currently for an E63) makes sense unless you are going to use a trusted indie, especially if you can time it so you get B-A-B services, and at least one of them includes a set of plugs and/or a transmission oil change.
In terms of total cost of ownership, the biggest single element is depreciation which is eye-watering in the first couple of years and then slows to just "very painful" level. The difference between dealer forecourt and trade-in prices is pretty large, even on a three or four year old car (typically £6-7k), so unless you want to be royally reamed don't even think about getting out of one in less than two years. Bearing in mind the high depreciation and the fixed service intervals, the cost per mile of ownership (if that concerns you) can be quite astronomic if you only do a small annual mileage. I solve that problem by driving around 13k miles a year in mine