I've built a couple of these over the years. While resistive loading a fan to control a fan's speed is not really the best way, it's easily done at home with a little smoke and solder. What you may want to watch for is the forward biasing voltage needed to illuminate the LED. Like on mine, I had blue LED's which are lit when on and red LED's when off. Both called for different voltages for their states and complicated the resistor mess on the back side while still driving the fan. Here's the Beta I still drag around to check WTF I'm doing when doing it still today:
Not pretty, but wasn't suppose to be really. Once you get an ugly mess like that you can translate it over to cleaner breadboards or even get more complicated and get into actual IC chips for fan logic.
A rheobus isn't really resistive in this sense. What we're doing here is placing external resistors to eat power for lowering fan speeds (or just making an LED glow) where a rheostat puts that back on the PSU [very basically]. The rheostat behaves like a resistor but doesn't eat the power. With those, you still want to limit the minimal power at the "0" point to be 7v vise 0v since that's the basic start up speed of the average fan.
To figure out the power rating: Ohm's law/P.I.E. Chart. Fans have ratings stamped right on them and you adjust your design accordingly. P(W) = I over R. A common molex will be able to drive MANY fans off one bay device with no problem.