Well, you don't really NEED speakers, but if you want a cleaner, crisper sound, you should get new speakers. As for the head unit, when you go to a shop, they should charge you around $100 dollars for instalation. $50 for labor, and $50 for parts. that is bull because the harness costs about $10-$15, and if you have the P10, it uses an ISO-DIN slot, which means you can just use the stock mounting brackets. And for the P11, you would prolly need a slot for the lower DIN if you have a single DIN head unit. if they install that "step down" harness (which cuts out the factory amp) it shouldn't cost $150. I say, do it yourself, save some money. I know that they do have harnesses out there that do cut out the stock amps, but you could just get the regular Infiniti harness, and re wire your speakers right from the head unit. By the way, what kind of head unit did you get?