My old man has a 520i SE E60. Had it for coming up to 3 years, bought it from Stephen James on a 53 plate with 31000 miles on it.

So far it has thrown 2 coils (apparently common). One of the Traction Control modules packed in so traction was constantly off and when he first bought it whilst AT the dealer the wipers and indicators and mirrors wouldn't work I think or something like that another module that controls them or whatever so that was changed instantly. This was all under warranty though.

As for the car, very very comfy smooth 2.2 straight 6 170bhp. Isn't underpowered in my opinion but if you are going to go for a petrol I'd get a 525i or 530i. My dad only bought a 520 because he is 60 years old, doesn't give two shits how fast it gets there as long as it cruises comfortably and is very spacious, decent amount of toys as standard. It's also a manual not an auto. When I've driven it occasionally it needs to be over 3000 or so rpm to properly get going. I think a derv is more suited to the 5 series but since I don't like dervs if I were to buy a petrol 5 series it would probably be 525i or 530i. I reckon a 525d or 530d again would be plenty. Depends on what you feel really.

To summarize, pretty reliable, very comfy, plenty of space, looks great, nice toys, won't bend the laws of physics but isn't a total dog.