Well I think common sense is the rule here. A priest driving a Mercedes is probably really bad optics. I don’t know any that do or would But they are not required to drive a beater either.
Frankly I don’t want my priest breaking down on the side of the road when I’m in the hospital waiting on last rites. :-)
As far as I can see, so does Pope Francis. A priest should have transportation that will allow him to perform his duties efficiently and safety.
My pastor has an Audi sedan, very safe and reliable. I think his family bought it for him. Some priests can walk or use public transportation or rarely leave their missions, and don't need to have a personal vehicle at all.
All of them - and all lay people, too - should be asking themselves, "Is this good stewardship? Is this apostolic?"
Could be worse.