If you put more than 5 volts into the input of a PIC pin you run the risk of killing it.
If you are using a voltage divider to get a signal from 15v down to 5v (max) but get a spike of 30 volts on the sensor input then you will be putting 10v into the PIC input pin which could easily kill it. You need some kind of circuit to stop this from happening. A simple zener diode with a series resistor would give the most basic protection - this would be classes as the protection definition. An opamp with a 1/3rd gain would be classed as a buffer.
Get the basics running first and read a lot on sensors etc as well as automotive electronics and you'll get there eventually.
As for the 18F vs 16F, the 18F is far easier to use, has less "gotchas" i.e. page and bank selects, enhanced instruction set, better internal oscillator support, hardware 8x8 multiplier, generally more program and variable ram and is generally a nicer PIC to use.
Since moving to the 18F series a few years ago I've rarely used a 16F series PIC.