Since this is a pretty open ended project there are lots of ways you can implement it. If you don't have a part called for in the instructions, find another way! I prototyped the whole thing with cardboard and tape before getting all the brackets and things I needed to make something I could hang on the wall.
So, things you will definately need:
Imp, Imp Breakout, Cowbell, Solenoid (or something to hit the bell), 2x NFET (I used a Sparkfun mini FET shield), Diode (I'll explain that below), power supply, Some way of holding it all together (I used wood, angle brackets, nuts and bolts, but anything should work).
Here are the electrical steps I went through: