Any reason you're not looking at Avant RS4s? I've always preferred them to the saloon because they're more practical and slightly more subtle.
I love a good E, but as with all these cars don't even think about one without a full MB warranty, the wallet damage they can rack up is painful without it. My dads 55 wagon had hundreds of little niggles in it, spending
way more time in the dealership than the E39 M5 that directly preceded it. It was slightly worse on fuel than the M5 too (20-21 vs 22-23 on a motorway cruise, averaging about 19-20mpg over the 2 years), but that's probably mostly the gearbox, which was the 5 spd auto, rather than the 7 spd the 63 models were fitted with.
The dealers round this way seem worse than the BM dealers, but that probably varies with location, they took over a week to diagnose a faulty sensor in the engine that sent the car into limp home mode because their fault code reader wasn't specific enough, I'm fairly certain they just starting replacing everything till it worked
So its not very reliable, not well built and not very economical, but I absolutely adore them, especially in estate form - you can fill the car to the roof with junk, then add a roof rack and it will still cruise and overtake effortlessly

One thing it was particulaly good at was getting between 50mph in 5th to 90mph in 3rd as quickly as possible, which made overtaking very safe and easy. It took 5 of us through to Holland with all the associated junk that goes with it, the Sat Nav worked, it never skipped a beat and was hugely comfortable doing it.
Its alright on A and B roads you know, but when you're late, its dark and there's a twisty B road between you and your destination you begin to notice the size and weight of the thing, so its just as well the brakes are epic, and epicly expensive to replace too.
I don't know whether it's just my imagination, but the E55 always seemed more fun than the BM, that was quite a serious car, an AMG is more of a point and squirt kind of thing. And you will point and squirt, because its sounds awesome when you cane it, yet the same as an x500 Mercedes when idling - a great sleeper
One thing they won't be is depreciation-free my dad had his from 07-09 from 4-6 years old, and its till managed to shed a silly amount of money in that time ~£10k iirc

that was buying at a peek and then the economy crashing and fuel shooting up again though, which undoubtedly had an affect.