My Dad had a 540i and Gavster had a 523i, both were great *
I had a 530i and it was riddled with issues, the most expensive being the torque converter not locking up properly. That was the point where I gave up. A real shame as it was a great car otherwise.
The first two were early E39s, mine was quite a late one. Maybe quality dropped off over time ?
Jon - the 550i is interesting from a rarity POV but £10k seems steep. You could probably get a nice F11 for that, a much better looking, better equipped car (except for the V8).
* but bear in mind this was 10-15 years ago when the cars were a lot younger.