1. I couldn't disagree more with the "aftermarket is better" guys. Maybe if your idea of "better" is that more people more blocks away can hear your boom-boom-boom from a mile farther away, but I think it's highly doubtful that an aftermarket system could match or better the "pure sound quality" of the JBL system, which was designed and built to sound correct in the Camry. Things like balance and ambience are built into the JBL system, which you're not likely going to get just by stuffing an amp and a few speakers into the car.
2. The Camry JBL system is by far the best car stereo system I've ever heard -- consider it "entry level" to stuff like the high-dollar systems in the high-level luxury cars. It's worth every dime it costs, and I wouldn't recommend buying a Camry without it.
3. Bluetooth is built into the JBL system -- navigation is just "something extra." Except where it's "something less," since it only has a four-disc changer.
4. There's not really much of a sense of "adding the JBL system to the car afterwards" -- it's a fully integrated system with the speakers, the head unit, and the amplifier under the passenger seat. Just get a car that has it, and don't think about trying anything else.
5. Yeah, I know what the aftermarket people will say, and what the aftermarket stuff can do. Sure, they can create "good sound" -- and sound pressure levels that'll damage your hearing so you'll be deaf before you're 50. The bottom line is that they can't be as well-designed to provide balance and ambience throughout the car like the JBL-designed system can. It does make a big difference!