Here's a list of Ryzen motherboards that people have tried and succeeded in using ECC with: http://www.overclock.net/forum/11-amd-motherboards/1629642-r...
Additionally, you can check the QVL for each motherboard, and then test it by hand.
Additionally, you can check the QVL for each motherboard, and then test it by hand.
*Edit - $750 is a lot for the cheapest threadripper + mobo. I'd be happy with 48 PCI-E lanes around $400 (CPU+Mobo).
$400 is really cheap even by consumer standards.