It has been reaffirmed many times that SketchUp and LayOut will use a single core no matter how many you have. So you should look for the fastest single-core performance you can afford if that is your main goal. As @Sonder said, though, if you run other things at the same time, particularly renderers, they may benefit from more cores.
I’d quibble whether the GPU is more important. Certainly you need one that provides good OpenGL support. Top-shelf gamer versions offer little added improvement for SketchUp. Experience is that Nvidia provide the most stable OpenGL support for SketchUp. But, just as with regard to the CPU, the best choice may depend on what else you want to run. As @RLGL says, some renderers use only the CPU (especially on Mac) while others can take advantage of a more powerful GPU. Again, Nvidia seems to be the most widely supported.