Hi Jbf,
Really sorry for your bad luck.
I have had my X5 Diesel and apart from the normal common small faults(tailgate rattle)
a worn front wheel bearing,a window regulator.its been a happy relationship.
The other Bmw I had was a M3-E30 Sport Evo from 1989 to 2005 ,and nothing at all to complain there.So I guess I been lucky.
Your smoke issue is rather odd,since your car has DPF and it shouldnt be smoking at all,inless its regenerating all the time you push it hard.Could be a software problem as well as a Bad DPF.