There are case fans with a temperature sensor. They already control the speed themselves depending on the measurement. The only thing they need is 12V.
I was actually looking at these on amazon but I mistakenly thought that the term "temperature controlled" included in the product name was just SEO bullshit, referring to the mobo controlling the temperature.
If I remember correctly (you'd better googel it to be sure), PWM case fans use a steady 12V supply, plus a pwm signal.
I'd buy an ESP, connect a temperature sensor, put the Tasmota firmware on it and be fine. The programming is a one liner then (in that weird tasmota rules language).
If it turns out that the fan needs a pwm "chopped" 12V supply instead, then you need to add a MOSFET and 2 resistors to create that.
P.S. that module from the other comment is better.
If you don't mind playing around with a Raspberry Pi and a few jumper cable, have a look at my repo. Half a year ago, I built a fan controller to regulate temperature and air flow in my server cabinet with 5V PWM fans. It is not finished and rough but it works.
After moving in a few months, I will have the time to work more on it.
They have a lot of products, including other fans and thermostats. I've had their media cabinet fans running 24/7 for 4-5 years now with no problems. Highly recommend it.
acinfinity is probably overkill for me but honestly very tempting.
I did see that thermostat pwm board on amazon. When you say a "standard 12v power adapter" do you mean as in... like a transformer plug with 12v output and the right amperage?
If you mean running the fans in 240vAC, Comair Rotron make fantastic fans for this voltage. If you mean a regulator circuit and any old 12vDC fan, sorry for misunderstanding.
I'm open to whatever solutions might be available, and these guys look like they make awesome fans, but I think I need something much less awesome and more integrated.
Basically, I just need a 120mm PC fan, supply it with the right voltage, and thermostat.
In that case, I'd probably be thinking of a standard power supply with molex output (they make bricks like this) for a 5.25" fan controller that ties in thermistors on the control side of the equation. I know that's not the typical, "I just use a raspberry pi and..." answer we're used to here, so take mine with a grain of salt.