This is about the best I can do...The operating System in Windows 10 Pro can be bought for under $10 on Ebay..fully legitimate. I have paired a 8400 with a GTX 1080 and 16GB DDR4 3000 RAM
CPU Intel - Core i5-8400 2.8GHz 6-Core Processor - $199.79
Cooler Master - Hyper 212 EVO $27.89
Motherboard MSI - Z370-A PRO ATX LGA1151 Motherboard $99.89
G.Skill - Ripjaws V Series 16GB (2 x 8GB) DDR4-3000 Memory $144.99
Storage SanDisk - SSD PLUS 240GB 2.5" Solid State Drive $53.99
Seagate - Barracuda 2TB 3.5" 7200RPM Internal Hard Drive $59.09
Video Card EVGA - GeForce GTX 1080 8GB FTW $434.98 Newegg
Case Cooler Master - MasterBox Lite 5 ATX Mid Tower Case $45.98
Power Supply Thermaltake - Toughpower GOLD 750W 80+ Gold $59.99
Total $1126.59
https://pcpartpicker.com/user/vMax65/saved/t3XyXL
This should provide top end gaming for quiet some time....
Choices can be made on the SSD and Power Supply to add or reduce price a bit. I would stick with a Z370 motherboard as they all have received BIOS updates for 9th Gen Intel CPU's so 9700K, 9800K and the 8 core 16 thread 9900K...coming in Sept/Oct.