Maintaining privacy in a (new) car might be difficult. Maybe by emitting white noise with an external bluetooth speaker so that possible microphones can't pick up voices easily?

The problem with privacy in a new car is that there is nothing much one can do about it. Even if it were possible by altering the cars' software (think of something like the equivalent of a browser extension for your car), this might pose a problem in case of an accident (the insurance will try to refuse compensation, because the cars' software has been altered and thus, the car is deemed to be not road safe).

One thing to aviod this problem would be to buy an older, used car. With that, there wouldn't be any issues regarding privacy, but mainaining the car might be more expensive, as older cars often have worse emission standards, which can result in higher taxes, depending on where you are located. Also, older cars might have a worse fuel effiency than new cars, depending on the size, weight and engine power of the car. An old economic compact car might have the same fuel efficiency as the latest doomsday street tank.

My personal advice: a used compact car, not older than five years (the older the car, the more repairs might come up, also there is a chance that a car this age already has a USB port for connecting a thumb drive or phone equipped with music), but with rear doors. Having two additional doors is an underrated feature that not only comes in handy when you pick up some friends, but they also serve as additional loading hatches that are accessible from both sides of the car.