The computer fan would work just fine. Also, the cheap humidistat is also acceptable.
I forgot to tell you that the guy who had the homemade unit kept it in a small room and ran a dehumidifier in the room. The humidistat wired in just like a light switch. Power came from his power source, The hot went through the humidistat and then to the fan and then the neutral came from the other fan wire back to neutral at the plug.
So he started with a slightly controlled environment with the dehumidifier, and then had his contraption in that room. Thus when the unit called for exhaust (too much humidity) it started and pulled in dehumidified air. I can not recall if he ducted the exhaust out or recirculated that air for the dehumidifier to treat that air.
But you are right on track with the motor and humidistat choices. Just make sure the humidistat can handle 120 v.
When you trace your holes cut make them 1/8 big. then push the insulation down and seal with expando foam. When dry cut it to fit snug. Then the motor should slide in with a good seal.
The speed switch would be just like having a ceiling fan on a rheostat. If it draws too much air, you can play with the speed to get it dialed in. For you air intake, make sure you use window screen to keep out insects and debris. You can get a moving louvered dryer vent which is cheap and install it inside the unit. (screen outside.)