There may be reliability issues with N62,but valve seals at 65 k?That is BMW after all.If the oil is going thru the seals and in the combustion chamber the smoke will be gray,not white.Excessive white smoke and water/condensation going out of the exhaust is reffered as a headgasket problem ,coolant going in the combustion chamber.But that is 99% result from overheating the engine.If it was never overheated it will be highly unlikely to be the problem.I would look for at least one more opinion if it were me.
Unfortunately it is no wonder 07 X5 will have some problems here and there.I feel for the owners of 2014 X5