Crusts answer is right for power supply, As far as getting and sending signals,
All (except a VERY few) the sensors run on 5V... The entire car has a system of 5V Ref wires running around... so thats easy.
As for the other stuff, its all 12V. Gas Tank Pump, ignition coils, airbag squibs, stereo, power windows/locks/seats, etc etc
That is in all the car's I've worked on. I bet there are some bmws or mercs that have oddball stuff in them.
Bewarned though, there is a SERIOUS push for all cars to be 48V in the next few years. (inevitable)
But for now, you SHOULD be fine w/ 12V... Unless your trying to do somthing directly w/ the battery or starter