To put music on, you would either have to download music from the internet as an mp3 or save existing cds onto your computer to convert it to mp3 and then upload it to the player.
Check that the headphones aren't fixed, if they're on a normal headphone jack you can easily change this over to a speaker jack. They're exactly the same.
If you get an MP3 with video capacity, she could view videos. Is this something that you want?
FWIW: Depending on her age I would be tempted just to get the ipod. The software is included and is easy to use and she'll never grow out of it. It's something that could do her from age 2 to 21. There's not many things that you can say that about!
The Barbie mp3 players are ok, DD had one and I bought one for a friend's DD last year. You can just drag and drop the mp3s on to it, once you have them on your computer, as it connects via USB, so you don't need to mess about with special software. However, if you already use iTunes for buying music, you can still drag the tracks to the Barbie player. You can do the same with audiobooks, as long as you get them in mp3 format (I use Audible).
They are also pretty robust, and stand up well to general use. I didn't use the ear buds which came with it, however I did get some children's headphones which were the big, over the ear type with built in volume limits. The speakers which plug into the headphone jack can be had very cheaply, but they do eat batteries.