You should go for a B450 motherboard. That Asus a320m-k may not have its BIOS updated for the new raven ridge CPUs (ryzen 2nd gen) and that means that you'll possibly need a ryzen 1st gen chip to update it.
Also A320 chipset motherboards lack in features and power delivery making them unsuitable for overclocking.
Motherboard:MSI - B450-A PRO ATX AM4 Motherboard ($69.99 @ B&H) Total: $69.99 Prices include shipping, taxes, and discounts when available Generated by PCPartPicker 2018-10-28 17:38 EDT-0400