I can see the logic of Manfred's way of working: if you get the results, no-one can tell you your doing it wrong, eh?
I am a prime guy and if the OP looks at this link, it is easy to see why.
http://www.dxomark.com/Lenses/Ratings
The top....what....70 lenses are Primes?
I am simply not prepared to pay for a lens which I know will result in every single image I take with it being optically compromised. I would rather miss the shot. Many others feel differently.
But Louise has to make that decision for herself.
Just because a lens has a large aperture, doesn't mean you have to use it and even at f/8, many primes are just better than many zooms, but I believe that by the time most photographers have been doing it for a while, they have both and use them both when appropriate.
It occured to me that Louise could stay with Pentax if she can get her hands on a K-3 II and a 35/2.8 limited macro and/or 21mm prime. I regret that I have no knowledge of Pentax zooms to offer. But the K-3 II with pixel shift resolution mode is a powerful beast with a great lens.