the OBD is a good idea, just do a serach on this forum with OBD-II, as i remind, some thread provide some already mad OBD-II ics.

That's true, you have to limit your voltage to 5 volt as PIC work on 5 volt too, once it's done some maths to convert into a readable value.

If you want to read voltage from battery with a range of 10-18Volts, you can use simple voltage divider resistor or using an op-amp with a gain of 5/18. One or another will give the same result. Wich can be the most accurate... i can't say as i never compare both but i guess that KISS engineering with the 2 cents resistors and, for sure, a capacitor to filter the noise (coming from car alternator and many much more source) going to the PIC adc pin will be the most efficient one. Don't worry too much about the temperature stuff, the PIC result will also differe by using one method or another. It's still to give an idea, if you're out of 0.1 or 0.2 volt it would'nt harm anybody. In fact it will always be more accurate than those analog in your car cluster ;o]

If you plan to do some OBD-II stuff, please choose something else than PIC16F84, there's a lot more efficient and cheap than this one.