The only thing that has stopped iTunes Music Store selling unprotected music was the record companies. That is now drawing to a close and I ahve been able to swop most of my many store purchases from protected mp4 (.m4p) to mp4 ( .m4a). Many digital music players of calibre that are not shackled to microsoft software now play mp4 such as most Sony Media Players and phones.
Your car player should support mp4. If it does not you can find software to re-rip mp4 to mp3, doing so may reduce the quality of the music but possibly not detectable to a car audio system. You would be better not deleting the original files as they will be a higher quality and keeping a seperate supply of your converted mp3's outside of iTunes and just copying them to your SD card as needed.
I have an excessively large music collection, the material portion of which began swamping the house interior many years ago. By embracing digital files for all new purchases I can keep 90 days worth of music in a box the size of a pack of tabs which is less than the space 4 cd's take. On the other hand as with the Doors Remastered cd/dvd reissue, the recent remastered collection of the catalogue of Kraftwerk and the CD set of each single in original sleeve released by the Clash I went online and bought the physical version in their limited edition collectors editions making a worthwhile item of home decor and eye candy