Hi brunox5,
I had an E39 525tds with 300,000km and it was exactly the same as you describe. I checked the glow plugs and 3 out of six had failed: once all glow plugs had been replaced, it started and ran perfectly again.
You can use a multimeter to check the resistance of the plugs, I think there's a control box somewhere where the six connectors all arrive in one place - you can probe from there (on my E39 this was under a filter housing).
That's where I'd recommend you start, anyway!
Good luck