Unfortunately wander spawn isn’t as sound-dependent as it should be.
The biggest danger to your car comes zombies organizing mosh pits in cars (they get really mad at cars due to the way sound attraction currently works). You can avoid it happening by sleeping underground or far from your car to keep it despawned.
Going fully electric can leave you stranded more easily than combustion engines, due to lower fuel capacity and slower refueling.
Muffler is good on any combustion engine, because it is essentially free. Don’t expect it to be enough to go stealthy, though.
Yes, different engines produce different levels of sound. It depends on engine’s current used power and in case of combustion engines also maximum power.
A V12 just idling with a muffler will produce more sound than a large electric going at full force.
A V12 without a muffler can hurt your ears (in earlier versions this would make you deaf).