I too have a 350z your number 1 problem with installing an iPad in it is the size of the dash. You first need to decide whether you want to mount it in portrait or landscape mode. Portrait is by far much easier in the 350z. You remove the radio and pocket above it and cut out the bar that separates the two and it is a near perfect fit. Landscape is a royal pita. Either you have it sticking out of the dash so far to make it removable that it just looks like *****. Or you have to make major modifications to the center dash piece and both sides of the dash where it mounts in to make room for the iPad. This means that you must pull out the center dash just to remove the iPad. I've done it all 3 ways and will get some pics of each up later if I can find them. Edit: look through my album on photobucket to see the ways I've installed mine http://s759.photobucket.com/albums/xx232/ITSJ4Y/
As far as the audio output you have a ton of choices and depending on your budget and need for sq will determine how you will want to do it. Are you planning on using a headunit or using the iPad as the head unit?