alexxander_foster:
Regarding Rds(On), I'm having a little trouble deciphering the datasheets. Ideally, I'd like to drive the MOSFET gate with the current at or near 100% flow directly from the PWM pins on the Arduino. Can someone tell me which graph to look for on the datasheet? Or what formula to use? Is it just Ohms law?
MOSFETs are primarily voltage based devices, so it's the voltage applied to the gate that determines how open the channel is between the source and drain. While MOSFETs do have a minimum current through the gate necessary to function, basically it's a very small threshold value. For most MOSFETs it should be well below the limits an Arduino output can supply. Now if you were using BJTs then the current of the controling signal would be a much more important consideration...
alexxander_foster:
Regarding back EMF, I'm wondering how I should address this. The Arduino is going to be mounted in the passenger compartment and the fans are a good 4 feet away. Am I being too literal by wanting to place a diode between the positive & negative terminals directly at the fan? Can I place the diode at the MOSFET located near the arduino?
The diode needs to be in parallel with the motor. That doesn't necessarily mean directly soldered between the fan motor's connection tabs, but if you put it else were you'll have to run the wiring appropriate to keep electically in parallel with the motor.