Car Audio - What to Purchase for 2004 Infinity I35?

I would like to replace my stock (2004 Infinity I35 Sedan) stereo with a "Pioneer AVH-X5500BHS" and a new amplifier with as little modification as possible. From what I have researched so far I will need a dash kit for a double din, a wiring harness adapter, an antenna adapter, and a steering wheel control adapter; correct me if I am wrong please. Most of these items I can find on "metraonline.com" or other similar sites, the stereo and amp will come from Amazon.com. My problem is the amplifier: The Bose that is currently installed is small and bolted on in a tidy little bracket under the top of the trunk, I would like to keep that setup if possible. The problem is that I don't know what form factor I need (to preserve the bracketing), what power for the correct sound, or how many channels I need (I would think 5, 4 speakers and a sub). If someone could point me in the right direction I would be grateful. I would like to avoid replacing the speakers if possible but I will upgrade them if the amplifier blows them out of the water. I don't need crazy bass, I'm going for whole quality of sound. Thank you ahead of time.