You don't need a CPU cooler for the 8400. The 8400 is a locked chip and the stock cooler will be more than adequate. You may want to add on later if it is too noisy, but the R6 is a very quite case and I don't think it will be an issue.
Since you have a locked chip I would get a cheaper motherboard as the main features for motherboards support better overclocking.
I would also go with 16gb of RAM for this build. This is a solid midrange PC and 16gb of RAM is optimal. It is better to buy it all now instead of getting 8gb now and trying to match 8gb more in the future. RAM prices are so high right now that there is not much of a difference between 2666 and 3200. So I would get RAM with 3200mhz speed for the 8400
That EVGA unit is not a great PSU. This Seasonic unit is better. If you have some more room in your budget, then you could look at a seasonic focus gold series that will be a little more efficient.
I love the case. The Define R6 is a very nice case.
PCPartPicker part list:
https://pcpartpicker.com/list/BcCGXP
Price breakdown by merchant: https://pcpartpicker.com/list/BcCGXP/by_merchant/
CPU: Intel - Core i5-8400 2.8GHz 6-Core Processor ($178.90 @ OutletPC)
Motherboard: MSI - Z370 GAMING PLUS ATX LGA1151 Motherboard ($99.99 @ Newegg)
Memory: G.Skill - Ripjaws V Series 16GB (2 x 8GB) DDR4-3200 Memory ($180.89 @ OutletPC)
Storage: Samsung - 850 EVO-Series 250GB 2.5" Solid State Drive ($99.99 @ Samsung)
Video Card: MSI - GeForce GTX 1060 6GB 6GB 6GT OC Video Card ($384.88 @ OutletPC)
Power Supply: SeaSonic - 520W 80+ Bronze Certified Fully-Modular ATX Power Supply ($49.99 @ SuperBiiz)
Total: $994.64
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-03-06 12:17 EST-0500