This is a good board that DOES require a BIOS update, but it has BIOS Flashback, so you don't need the CPU, just a USB:
Motherboard: MSI B450 TOMAHAWK ATX AM4 Motherboard ($114.89 @ OutletPC)
Total: $114.89
If you want to go completely new chipset with no BIOS update at all:
Motherboard: Gigabyte X570 GAMING X ATX AM4 Motherboard ($166.98 @ Newegg)
Total: $166.98
Also the B450 chipset or X470 chipset are good for the 3700X but require BIOS updates (asides MSI MAX series), however you can find sellers that sell them out of the box ready, so you could always consider this, I haven't listed them here because it varies on the seller.