"Something you didn't think of, which I've listed below." It was a close choice between that and "Best low light performance for my budget."
It needs to be a significant improvement over what I already have. My Pentax progression went from Kr -> K5 -> K1.
The Kr was my return to photography. It was my first DSLR and a huge upgrade over my digital point-n-shoot, and over film.
The K5 was purchased primarily because of its low light and dynamic range performance. I was getting into astrophotography and the K5 was markedly better than the Kr.
The K1 offered multiple improvements over my K5. Better low light performance and several new astro-specific features. No AA low pass filter, improving sharpness for all genres of photography. Improved AF for action shots.