My 2.5DSE has done 196k miles, and still going strong with no engine problems. I'm currently doing around 1000 miles a week! Although it has been serviced every 6k since new.
I also had a BMW 525TDS with the same engine in, and it had 186k on the clock when I sold it.
Has a timing chain like the petrol so no need to worry about belts.
I believe this engine is prone to over heating, but as I understand it, that usually happens when the engine has been neglected.
On hard acceleration these engines do give out what I would consider to be quite a bit of smoke by modern diesel standards, so I wouldn't worry too much about that unless it looks excessive. On tickover and normal driving you shouldn't really notice much smoke from the exhaust.
Like everything I think, if you look after it, it will run well, but if it hasn't been serviced regularly it will have problems. Just look out for one with a FULL history.
Matt.