So while Im off to read the datasheet on IRF540N and watch some MOSFET for dummies video on youtube, how about the signal input?
Will I need large inline automotive-type resistors? and is there more to it than that?
Will the normal running of the circuit that supplies the signal be affected by adding to it?
No, you will not need huge cement resistors. Now you have a MOSFET, turning the motorbike lights on and off would be the same as blinking an led on the Arduino. You can just use the "analogWrite" function.
I intend the standalone IC to run off the motorcycle battery, are the power requirement the same as a full Arduino board? and would the ground pin run to the motorcycle frame/chassis?
I'm not so sure of the power ratings, but you can probally find it easily. You can then select an appropriate valtage regulator. The voltage regulator will have three pins--input valtage, common ground and output voltage. You connect the input to the positive lead of the battery. Connect the common ground to the negative lead, and the output gives you the regulated voltage. You connect the ground to the common ground. The voltage regulator might get hot as well, you might also have to screw on a heat sink.