Depends on what you are wanting to use as inputs.
If using an iPhone, for example, and you just want CarPlay, then only lightning to usb cable hooked up to usb1, is all that you would need.
If you want to use AppRadio mode, then you will need Apple Digital A/V Adapter and a hdmi cable and lightning to usb cable(for charging only)
If you plan on using a thumbnail drive, or solid state drive on usb2, then I would purchase an extra usb extension cable and run to glove box or something.(the unit comes with one extension, but you do not want to have to remove head unit to change devices hooked up to usb ports.
If you are going to hook up other input like a digital tv tuner, then all you would need is rca cables hooked up to the A/V input.
So fully functional could mean a lot of things.
I have nav addon, digital tv tuner, siriusxm, SDCard slot, hdmi input, and both usb inputs being used. the only input i do not use is aux, but have the cable in the input with rca leads in glove compartment for video camera on dash when i want to use it.
Just remember, it is either CarPlay or Appradio mode(you cannot switch these two modes without switching cables and switching smartphone inputs in the settings)