Id steer away from mp3 format as a demo as they are compressed. Get a genuine CD disk of something you know has been well produced.
That’s bollocks. In a car, with wind and tyre noise, it’s totally irrelevant; a recording on a metal cassette tape from CD would sound as good, an MP3/AAC at 320 Kb variable bitrate is virtually indistinguishable from a CD or lossless version to the greater majority oh human ears, in a car you’ll never, ever be able to tell the difference; I’ve done comparisons between 320Kb and lossless versions of the same track, and couldn’t tell them apart.
Because of all the background noise, unless you’re driving a Roller or a Bentley, classical is a challenge, the wide dynamic range, with lots of subtle quiet bits, can make it tricky to reproduce in a car, when the loud bits suddenly come in it can be a bit of a jolt, unless you’re listening to lots of early 20th Century English pastoral string music. The 1812 might be interesting; there’s a version, I think by the Atlanta Philharmonic, which used real cannon!