You actually don't need a power button, just connect the two pins on the motherboard labelled "power" with a conductive material like the tip of an usb key and it'll start.

As for ubuntu I usually mine on windows, depending on the cards the drivers are usually better there (amd has a blockchain specific driver for instance), never ran into any troubles with that. I suppose that for high gpu builds ethOS might be usefull though.

Would have saved you 49 dollars wooh a third of a second gpu