I think I'm going to need something to convert the 24V DC output to the input current for one of the Arduino's above ... suggestions? The Uno operates on 7V to 12V DC 1-2A I think?
I'm not sure what Arduino board to use from the list above (guessing Uno), suggestions?
The basic purpose of the circuit is to increase output to the fans as the temps increase. Now I realize there are probably fan controllers that might be able to do this for me, but I want to have my own circuit (that I may update later to include temps display and power curve adjustments and humidity adjustments).
Wrong. You need converters, likely buck, converters delivering 12 volt to the fans and the corresponding 5 volt to the controller, UNO or mega. Micro and Leonardo I don't have in the memory. Watch out if they need 3.3 volt Vcc.
This is a suitable project instead of buying expensive ready made controllers.
Because You will run two relay modules and the sensors I recommend a 24 to 5 volt buck converter delivering 5 volt.
The internal 5 volt converter on an UNO, a mega, will be at the edge to overheat if 12 volt Vin is used.