Diesels don’t like repeated short trips, newer ones doubly so.
My other thought was, do you really want to do 150,000miles in a B-class?
[edit] Iwas thinking the tiny a-class, the B is focus sized isn’t it?
Personaly, I’d get a new(er) car for yourself and run that to an astronomical mileage. Then let your Mrs get herself a new (amaller) car when she wants it. A 5-series is pretty big and I’m assuming small mileage = short trips where ease of parking is more usefull than comfortable motorway wafting. The only downside of this is your plan would be most likley to kill the car simoultaneously with high mileage, rust and other age related issues. Your missus plan will kill her car off with rust most likley well before it reaches a similar mileage.
The ideal I suppose would be buy her a small car thats done loads of motorway miles, and yourself a low mileage luxo-barge and they’ll probably die at the same time.