Going to be used for car maintenance/restoration work and around the house repairs.
Core functionality of a multimeter:
Measures Voltage - the majority will cope here, as you typically are only dealing with 0 - 12 volts across most areas. You aren't going to need it to measure high tension things such as spark plugs, as you can check them using far simpler methods (such as a spark from engine block to removed spark plug)
Measures Current - currents in automotive circuits and audio circuits can be very high, so check your multimeter has a very high internal resistance and high current capability.
Measures Resistance - you'll probably find the most useful part of this is an audible continuity checker for checking cables deep in the bowels of your car
Frequency - less common on multimeters, but can be useful when working out speed/timing issues
Also make sure it is water resistant, and ideally comes in a rubber housing (as per @CodeBlend - Fluke are generally a robust and accurate brand to go for)
Anything more, and you're likely to need a computer with adapter and software to look at your ECU.
I know a couple of bits and pieces but not enough to recomend, here is a forum with some people that sound like they know thier stuf;