You will need the box, called a port I think and one cable to hook your depth finder to the box, you will take the cord off the 999 and hook it to the box from the 360, your new Ilink will come with a cord for it, so you will need one for the depth finder. Also, if you plan to connect the unit at the console you will need a cord for it too.
The Ultrex is a cool dude, been impressed by it but enjoy the stow and deploy too much on the Ulterra to give it up.