What kind of SLR and lens do you want to buy to take pictures of the sky?

There is no requirement for the SLR body, just use a general lens group. The key is to find an expressive cloud scene. You can use the Polarization Filter (CPL) to deepen the color of the sky, making the blue sky deeper and the white clouds more prominent. You can also shoot the sunset glow at dusk, which is often very expressive. Example of CPL effect

Sample, dusk sky