If you wish to add the oem changer, fine, but install it yourself. It's not hard, and more importantly, assinine to pay that kind of $$$ to the stealership.
If you wish to add aftermarket speakers, amp, and sub; forget the oem changer and replace your head unit with a quality aftermarket head unit. You could purchase a great deck for well under $500 (the cost of your changer), and would realistically the only way to add other aftermarket components.
Yes you could leave the oem deck, and add the other stuff, but that is certainly bassackwards. The head unit is the beging of everything, hence it being called the head unit or source unit.
A line level converter would be necessary if you wish to add an aftermarket amp to the oem deck, and this would start the signal flow in a very dirty fashion leading to the dissemination of a dirty signal througout the rest of your brand new aftermarket audio system.
Either add the oem changer and leave it, or buy an aftermarket head unit, and then add whatever you want. I recomend... Head Units
Nakamichi- High End
Alpine- Mid Grade
Kenwood- Good Entry Level Amps:
Xtant or HiFonics- High End
Precision Power or Infinity- Mid Grade
Kenwood Excelon- Good Entry Level Speakers
DynaAudio or Diamond Audio - High End
Infinity, PolkAudio, MBQuart- Mid Grade Subs
Xtant, Hart High End
Infinity, Kicker- Mid Grade
Your choices are fine, but the above are all prefered. As for improving ventilation, any system built right doesn't need any cuting or venting to add bass. I have one 8" sub, yup one 8, and it is more than I need, and I have modified nothing.
Just opinions from someone who knows, merely my $.02