The BMW x30d engines are fantastic (both 530d and 640d are great). So are the Mercedes E350CDI engines (OM642). You can't go wrong with either. I think the BMW's feel much more modern inside though, with much better infotainment.
For the type of car you're looking at, I'd be looking at a diesel engine (almost vomited when i said that) - they are effortlessly wafty, get up to speed without any drama, the 6cyl don't sound bad at all, return good MPG and are so smooth to drive. I'd find it hard to recommend any sort of petrol over these two diesel cars.