I think Valve and the Valve Linux Gaming community will put out a list of compatible computer parts which work for both Ubuntu Linux + Steam for Linux at running games in Medium to High Quality video settings, so Ubuntu Linux users and 3rd-party companies will less likely run into un-compatible video cards, and sound cards which don't work on Linux and Steam Linux.
I think most Linux-only computer makers like System 76 will make their PC compatible with Steam Linux, and sell gamepad controllers which work for Steam Linux.
I bet they will most likely use cheaper computer parts like video cards from Taiwan like Foxconn, Asus, and other companies which sell for less than American brands because the Taiwanese use cheap labor from China, India, and other countries where people make a few dollars a day, so the savings gets passed on to the consumer.