A few thoughts about your question about sharp screenshots:
I'm playing on standard resolution, but I really wonder if there's much benefit from going 4K in Combat Mission. The underlying graphics are not that detailed, and there are many glitches, so you're just going to see all the shortcomings amplified, I think. Also, making the user interface even smaller is not going to be helpful.
What I could see being a benefit is buying an extra wide monitor. Because that might help you get an overview of more of the battlefield at the same time, increasing your situational awareness.
In any case, CM is not so much restricted by the GPU as the CPU. I think most GPUs these days could handle high resolution CM gaming, but if you want to increase performance, go for the CPU that has the highest single-core performance.