I know you said E55... but have you considered the C55? Much simpler in design, less things to go wrong and it is probably the most reliable of your choices. I only bring this up because i have one right now, and i came from a 335i. Great choice in my mind! Take a look at them.
this isn't mine, but you can see they look very similar... and they go for cheap nowadays around 15K or so...
