The K-7 isn't a dog. You can take great photos with it, and if you search on this forum, you can find a number of examples of high-ISO photography with the K-7 that is surprisingly good.
I upgraded from a K-x to a K-7 and I don't regret it. The K-x let me be sloppy with exposure — I could underexpose and push the exposure in pp without too much pain. But the K-7 forces me to be a bit more deliberate and thoughtful. I think I'm taking better photos as a result. The K-7 has great color and contrast, and if you expose properly, you can get good photos through ISO 3200.
If I were a pro, I would go with the good lens and K-7. It is truly amazing the improvement that a great lens can make in contrast, color, smoothness, etc., even at the same ISO and aperture as a lesser lens. I used an M 50/1.4, a M 135/3.5 and a Tamron 28-200/4-5.6. The shots with the primes were head and shoulders above the shots with the (admittedly cheap) zoom.
Here's the set, if you're interested. Again, I know that my photos aren't pro quality; I'm still a relative newbie.
That said, as an amateur who does a lot of shooting indoors, I am probably going to sell my K-7 (and some other items) and buy a K-5. I really enjoy indoor, low-ambient light photography, and the K-5 will be the better body for that.
Just out of curiosity, why get a zoom for portrait shooting? The subjects aren't going to be moving. I suppose you could recompose at different crops quickly, but a good prime will best a good zoom in final quality, even if you have to do the foot zoom.