There are a number of threads here that address 5,1 upgrades, so they should be your first source rather than starting a new thread to rehash what has previously been said. That being dais, Your are better off upgrading the GPU to one specifically recommended by Apple for Mohave unless you have a different use in mind. I use my 5,1 mostly for video, so my GPU has two HDMI outputs to which I have connected TV monitors, and allow surround sound output. Your needs might be different.
You should get a two processor model and not the single, because the single can't be upgraded without a new sled, which is really expensive, if I remember correctly. You can upgrade the CPUs fairly cheaply, so don't worry about the CPU in the 5,1 you buy.
Back to the GPU, you might want to rethink dual GPUs. The pci-e slots really only permit one GPU because of the fan placement, which would be blocked if you add a second GPU. I had a bear of a time putting in both my GPU and the SSD sled, as well as an eSATA card and USB3 card. Because my GPU has dual HDMI out, it isn't necessary for me to have a second GPU, but if you do need a second, you might have to look around the forumsor on other MAc sites that can help with the limited space.
