Seagate hard drive aren't known for reliability. I switched to a Hitachi. Just be aware Hitachi's are pretty noisy. If that's a deal breaker get a WD. Although they don't have quite as good of a reliability history.
The ADATA SU800 is considerably faster than the Kingston A400 and has a little more storage space. A Crucial MX500 would be even better.
Motherboard: Be aware an X370 chipset motherboard may require a BIOS update. Meaning you'll have to wait for AMD to ship you a boot kit if your motherboard does not arrive already updated. If you don't want to do this get an X470 chipset.
X370 doesn't offer anything you need. I'd rather a good quality B350 than the cheapest X370 any day. Note that B350 has the same BIOS issue.
RAM: Many RAM models have trouble with Ryzen. I switched to the Corsair because it is on the QVL for the ASUS motherboard I chose. You can look through the QVL for something cheaper. QVL is the Memory support list.
CPU: If you are going to OC. There is zero reason to spend extra on a 2600x.
PCPartPicker part list /
Price breakdown by merchant
CPU: AMD - Ryzen 5 2600 3.4GHz 6-Core Processor ($189.99 @ Amazon)
Motherboard: Asus - STRIX B350-F GAMING ATX AM4 Motherboard ($111.99 @ SuperBiiz)
Memory: Corsair - Vengeance LPX 16GB (2 x 8GB) DDR4-2666 Memory ($179.99 @ Newegg)
Storage: ADATA - Ultimate SU800 256GB 2.5" Solid State Drive ($61.99 @ Amazon)
Storage: Hitachi - Ultrastar 7K3000 2TB 3.5" 7200RPM Internal Hard Drive ($53.99 @ Amazon)
Video Card: MSI - GeForce GTX 1070 Ti 8GB Video Card ($479.99 @ Newegg)
Case: NZXT - S340 (Black/Red) ATX Mid Tower Case ($59.99 @ Amazon)
Power Supply: Thermaltake - Smart Pro RGB 650W 80+ Bronze Certified Fully-Modular ATX Power Supply ($68.03 @ Amazon)
Monitor: Acer - GN246HL 24.0" 1920x1080 144Hz Monitor ($194.62 @ B&H)
Total: $1400.58
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-06-14 18:31 EDT-0400