What hardware should I upgrade to have a good modeling and rendering machine that won't hang or stutter. I currently have Maximus VIII, 6700K, 8GB 2400MHz RAM and GTX 1060.
You have a pretty solid build, but there are a few areas you can upgrade to reach optimal performance for rendering, the main one being your memory configuration.
If possible, I would toss/sell your current kit and move to 16GB DDR4 at least 3600MHz at a minimum, if you want to be optimal I would go 32GB.
Another optimization would require knowing what software you will be using for rendering. If it takes advantage of the GPU, an upgrade from the GTX 1060 6GB to something with 8GB of VRAM (or higher) like the 2060 SUPER or RX 5700 (or better).