I will assume you've considered safety concerns of trying to build your own vehicle. Arduino is not designed for automotive use, but it could be used for prototyping. Just make sure you do significant testing.
As for the other parts:
Speedometer: Easy, use a rotary encoder on the wheel? You can double check with a GPS, but that seems the simplest. You can get magnetic or optical ones, and I'm surprised you aren't already using them to intelligently control and monitor the speed.
Voltage meter would be easy, just buy an analog Mux that goes between the various voltages you want to read and only use one ADC. It would probably need heavy filtering.
Ammeter: you can use either a hall effect or a shunt resistor. Fairly simple. Should probably be tied into the motor controller anyways for safety.
Temperature: There are plenty of sensors, just make sure you get one with the right range. It might be easier to get a digital sensor (i2C or something similar) to avoid noise issues.
Headlight: Again simple, especially if you find a premade module you can just switch on and off with a transistor or relay.
Other stuff:
System Shutdown: this is a must, as well as an E-Stop that will kill power to the whole system no matter what. Use a relay/solid state relay. The E-stop is usually that is just a massive switch wired in series with the battery. However, make sure you display warnings before you just kill power.
Music/Bluetooth: This will require an external device. Its entirely possible, however arduino is non-ideal for music. There might be a complicated way to integrate this, but you do not want to dedicate much processing power to this over essential functions.
Garage Door Opener: This is an eternal device. The easiest way would be buy a working one, and replace the button with a transistor/relay controlled by the device.
Keycode: This is just software