After some more thinking and research, I have decided to modify the project again.

I recently came upon the realization that using the large TFT LCD described in a previous post would introduce some problems. A larger and more complex LCD like that requires a larger and more complex PCB to support it. This will increase the cost and time necessary to design/build it. Since it is a "higher" resolution graphic LCD, the program complexity and overhead will also increase. For a project like this, graphics would be nice, but are not necessary. I did some more searching and found this nicely sized character LCD, very similar to your standard 16x2 char HD44780 LCDs.

As for what is going to be displayed on the LCD, I have decided to implement a menu system. For example, one could choose from FM frequency control, music control, settings, and likely more. Because of the reduced program requirements, I could easily get away with using an Atmega32u4 or Atmega328 as the main microcontroller. However, I would still like to try the LPC11U35FHI33 first, since it is a little cheaper.

I have also decided to do away with the separate power supply box, and integrate the power control circuit into the main device instead. This will reduce the overall system complexity, and also potentially reduce costs. I am planning on implementing the ability to enable/disable the voltage sensing capability, to allow for use on cars that don't have the problem mine does.