Thank you. That helps.
I do not plan on varying the fan speed. No PWM, just on/off. I know I'll be using a standard 120mm PC case fan or CPU fan (whatever I can find that's QUIET and inexpensive), something that moves about 70 CFM, and while most of those support variable speed use these days I don't see why I'd really need that. Worst case, that'll be a version 2.0 improvement I can make.
Looks like SparkFun makes an "Ardumoto motor driver shield" that is what I need to switch the fan on/off while powering it separately. Here's a new block diagram:
Anybody have any ideas about the LONG sensor leads issue?