Yes you should be able to add the music you already have on the computer. You can also download audiobook and regular books if they are available through your library through overdrive for no cost. We listened to Rick Riordan on the way to florida.
You can check out itunes each week for free apps though many of those come with advertisments. You can get a lot of others for $.99 cents
We have gotten gift card deals at christmas so we buy a few extra.
Some of their current favorites
Smurfs
temple run
temple run brave
any angry birds
mini pets
where's my water
Also our kids do not do facebook so we did connect them to game center so they can have friends on some of the games. (If she wants a smurf buddy PM me)
We do not have CC on file with Itunes and only use gift card added to the account so kind of self regulating if the buy something not realizing it is costing real money. Example smurfberries on the smurfs game (they are very clear some apps aren't) Our family does do smurfs

Our DDs do not have their own but use DH and mine though Santa may be generous this year. We have not had a problem with this but our DDs do not have the password for itunes.
Some educational games Stack the States, Stack the countries Not sure of the age of your DD.
I would try and get one with 32GB of memoryif possible. Mine like to have a few movies on mine and they take up a lot of space. itunes will occasionally have free tv episodes they like. Also if they like to take pics and video it takes us some room though you can download them to the computer.
You can get a refurbished one from apple that will come with the same warranty as new and save a few dollars. I would get the current generation as our kids like to facetime each other and a few of their friends.