I bought the basic Edge 500 that came with just the bike mount, power cable and USB cable. I didn't need the Garmin speed/cadence sensor because I wanted to use the Bontrager Duotrap sensor instead, and I don't need a HRM.

I'm pretty sure (but not 100%) that the Garmin speed/cadence sensor that comes with the Edge in the package deal is the same one you can buy a la carte.

BTW even if you get the package that comes with everything, you might still need to get a spoke magnet.