Are you planning on using the navigation unit to watch DVDs? Also, do you want to have surround sound or are you just fine with 2 channel sound?
If you're just fine with 2-channel sound and will be using the navigation unit for dvd playback, all you'll need is an adapter that converts the IP bus input on the head unit into an auxilliary audio input, such as the CDRB-10 or CDRB-20.
If, however, you do want surround sound, or want to be able to control the dvd from the head unit, you might want to think about stepping up to the AVX-P7300DVD. It has an optical digital output to connect to a surround sound processor, and since it has the IP-bus connection, you can plug it in to the head unit and be able to control it from there.
We kinda got off the original topic of the capabilities of Pioneer's IP-bus. My understanding is that in order to add on more IP-bus compatible components, you need to have an proper input/output chain going, and as long as that chain is kept there's no limit to the number of components that can be hooked up.
For example, look at the DEX-P9: it has the ip bus output designed to go to the DEQ-P9, but it also has the IP-bus input. A component like the surround sound processor (DEQ-P7000) could hook up to the head unit's IP-bus input, and then on the processor's IP-bus input could go the AVX-P7000DVD. The chain would not be able to go any further than that because the AVX-P7000DVD does not have IP bus inputs. So for example, if you wanted to add XM to your vehicle, the xm tuner (which has IP-bus inputs and outputs) would have to go BEFORE the dvd player in the IP-bus chain.
I'm sure there's a limit to how many things can be hooked up at once but as far as I know the system will support at least 5 separate components without any problems, and I don't see how you would ever need to hook up more than that. (changer, xm, dvd, surround sound, navigation, anything else?)
One of the nice things about the Pioneer IP-bus system is that all the head units are designed to have this unlimited expandability. They have "source" buttons instead of a separate button for each different source of the unit. For example, some manufacturer's head units have a "CD" button, an "FM" button, an "AM" button, and an "AUX" button, so with such a system you would be limited to those few buttons. But with pioneer, you just keep pressing source until you flip through all the different components that are connected to the BUS chain..