Which SLR should I start with at this stage? Great Spirit and Talent Progress

The gap between the whole set and APS-C can't be made up by the lens.

No matter what the purpose is, it needs to cooperate with a standard change. As for whether it's 24- 105 4L or 24-70, that's your money.

On this basis, consider what lens you need. I don't know if 30-40 you said is the equivalent focal length.

Stay away from fixed focus, big ternary is used for work. If you don't think it's enough, then I can only say that your technology is not home, not equipment. And fixed focus is used to install B.

As for sharpness, distortion and color, I suggest learning more in the future. In the digital age, you really don't have to care too much. What you need is enough functions to meet your requirements, rather than focusing on image quality. If you think the function of this camera is enough, then you don't need to upgrade. The reason for your upgrade is mainly because of the light, not because you really need it.

Finally, don't take it for granted that the quality of big ternary is not as good as the fixed focus below 3000. I can clearly express you here. No matter Nikon or Canon, Dayuan's quality is absolutely enough. What you lack is mentality. Finally, stick a 24-70 2.8G film, the light is on, and the focus is on the second tube. Then you take your Pentax camera+fixed focus and compare it. Look at the picture with the size of 100%, which is only better than sharpness and glare resistance.