No known problems. Battery lasts up to 10 hours.
Car adaptors are available. There are hundreds of games available. As far as memory gose no extra is needed to play any games that I know of.
As far as using it as an mp3 player goes you will need to buy an M3DS Simply click here
which has the software necessary to play mp3 files as well as movies & text e-books plus DS Roms.