From your references to 'monetary loss', are you thinking that you're actually going to make any money out of coin mining? I think you'll find that boat has sailed long ago.
You should be able to PWM control ordinary brushed PC fans, and you can use one or more DS18B20 temperature sensors and potentiometer inputs to control the fan speeds using whatever algorithm you want, and you can drive a relay to operate a relay which shorts across the 'power' button if you want. I'm surprised that any of this is necessary, though. It's been years since I bought a PC without automatic CPU temperature sensing and fan control, and I think you're reinventing the wheel. But what you're describing is not very complicated, if you decide that's what you want to do.