Of course you can buy a Kia with much better warranties, it will take less fuel, have larger space for luggage etc etc, but all is not black or white.
I would say no five series makes sence at all if it is value/£.
In my case the interior of the five series is WAY better than your plastic Z4. It is also way better than a new 3series or a X3.
It's not that we have a Kia CD and a 520D in one corner and hundred miles away is the 530D along with the Rolls Royce and Range Rover.
The 530D is still a Diesel and doesn't sound well at all standing still. It will also shake and vibrate lots more than your Z4 35i.
I would say, characteristic wise the 530D and 520D is closer to each other than a 530D and 535i.
If it is all about emotions, get a 550i, there is a refined engine.