Can you download music onto a blank DVD-R disc and play it in a DVD player?

Yes, you can, as long as you upload the music first.

It will depend on the capabilities of your car CD player. Some older ones do not recognise the CD-R discs or DVD-R.

If your car can actually play DVD's it is highly likely that it will be backward compatible for recordable CD's.

You may have to record the disk as a CD using WMA file format.

Check that your player can play MP3 files, then more tracks can be used in MP3 format.