My carrier MetroPCS (which doesn't have great rural coverage) and I believe other carriers such as T-Mobile offer free, unlimited streaming on certain music apps. I use Google Play Music almost incessantly and it doesn't count towards my data. Depending on where you're located and where the coverage maps are you might check out these kinds of plans.
/u/Tired8281 makes a good point as well. A simple, cheap PC with a touch screen mounted on your dash would make a great distribution system. Put the main OS on a flash drive or SSD and either mount your storage disks with shock proof bushings or suspend them with pantyhose or some form of stretchy fabric. I had a buddy who shoved his main drive (pre SSD days) in a length of hose and suspended it like a hammock in his vehicle PC case. Thing ran for ages.
rubber harddrive mounts which have a small piece of runner between the harddrive and the screws.
Something like this : Link