If you think it might get abused, get an ipod shuffle, 4gb version ($79). If you think you will use it for other music listening at times other than when you're getting shot at, get an ipod nano, the 16gb version. I have a previous gen ipod nano and I love it, but it is an 8gb version and it filled up before I could get everything I wanted into it.
If I had to buy again, I'd get one of the large capacity ipod nanos. But if you might crush the thing diving over a barrier because someone is shooting you, then an ipod shuffle would be a better idea since it is really small. There is also a stainless steel 4GB ipod shuffle for $99, if you're really paranoid about crushing it.
For either of them, I wouldn't worry too much about battery life... The shuffle has about 10 hrs playback, the nano has about 24 hrs audio playback, and they both charge fairly quickly so you can probably charge it enough just from plugging it into your car while driving around.