This system is all about maximum lifespan and stability. The MB/CPU/RAM/SSD are only a fraction of the overall cost. Because of this I went with the higher end. It was going to be a Intel i9, then i7 7700k was tested, then i5 6600k. The i5 performace perfect and the is priced lower than the 7700k.
There is absolutely zero difference between i5-6600k and g4560 in terms of "maximum lifespan and stability". If anything, g4560 will run a little bit cooler, consuming tiny bit less power. 6600k is a waste of money on a mining rig. And i9 is just lol. The only reason I commented on this is because you said that "low end standard CPU's, ie G4400 will not work beyond 8 GPU's, 9 MAX on windows". So I was curious about why they will not work. G4560 works.
Did not test G4560. The G4400 just did not have the processing power to handle 9 GPUs. they system would blue-screen. It took for ever to enumerate the GPU's and when you remote into the system all input lagged. Since the majority of the cost of a system was in GPU's, i did briefly consider the i9, agreed LOL. My cost on the i7 7700k is less than retail (of course) so lets say less than $330 and the cost of the i5 6600k is less than retail, say $250. The 6600k matches or outperforms the 7700k, taking into account cost & TDP.
Three months of testing various components & configurations, tweaking, technical support with all the vendors. Again the MB/CPU/RAM/SSD are only a fraction of the system cost. Adds maybe 10 days to the ROI.
As to AMD - I would would not touch one. I wouldn't even put them in the system if given to me for free. nVidia was selected with keeping the future in mind. You don't even have to mine coins in the next few years because there will be a high demand for Cloud AI, Smart Cities, Self Driving cars. You could sell Computer power to schools or anyone else that is looking to push their CUDA code to a supercomputer. This cannot be obtained with a AMD at this time.