Personally, I would start with a motherboard that can accommodate at least 6 GPUs, a good power supply, and a single 1070 (or AMD if you prefer...depends what you want to mine).  Build a solid foundation and you can add GPUs as you wish down the road.  It doesn't have to be a monster miner all at once.  Just get your feet wet with the simplicity of a single card and work your way up.