That is a good motherboard, and the cheapest and best CPU for X99 which has 40 lanes is the Intel i7 5930K 3.5 GHz.
which is also the same CPU NVIDIA uses for their pre-built machine learning workstation.
I am building a new PC with one of those now and it is a great CPU, well worth the price. You can comfortably overclock to 3.9 GHz with no issues related to temperature.
If you are using the Titan X you can also consider Windows 7/8.1 OS since you can put in it TCC mode which “hides” the display capability of the GPU from the OS and has slightly higher performance than possible with the native Windows WDDM display driver. IMO it is easier to get CUDA installed and working quickly in Windows vs. linux, but if you are already comfortable with Ubuntu then stick with that OS.
You will need a cheap GPU for the video display and the PCs I have built usually will use a GTX 750ti for video display and the GTX 980/Titan X for compute. So yes it is a good idea to have more than one GPU in the PC build.
Get at least a 512 GB SSD like the Samsung Evo 850 pro, or maybe even two if you would like dual-boot. Those are cheap enough these days to buy more than one. The 1 and 2 TB version tend to be less reliable than the 512/500 GB versions.
You should also use a liquid cooler for the CPU, and those can be a pain to install with some PC cases. Get a full tower and check that the liquid cooler fits into the particular case.
Oh and get at least a 1200W high-quality power supply so you can support multiple Titan X GPUs.