If you're only looking to haul MP3's, I very highly recommend an iPod. Not only will it be cheaper, but highly likely (in the approximate price ranges) it will have a larger hard drive than an older laptop.
The battery life is variable. My friend's 1G 5GB still has almost its original battery life even as old as it is. Taking care of it helps (and regularly discharging fully helps even it being a lithium-ion). Either way you're likely to get a lot more battery life out of an iPod than an older laptop, and it can plug into the wall just like a laptop can if necessary.
However, older laptops can be used for other things as well. I saw a free Thinkpad 760e on my school classifieds; too bad it was taken ten minutes before I responded. I was planning to use it for an ODB-II monitor for the car had I gotten it. Just a thought; but if you're using it solely for MP3's then a portable MP3 player would make much more sense.
