Get one directly from Apple either through Edu discount or refurbished discount. Pick up warranty a month before it expires if she still has it.
Being in a college dorm, she'll hopefully learn how to share music & files over servers wirelessly. At the very least 1GB of RAM (NOTHING LESS!), Superdrive (DVD burner), and a mini-portable hard drive. I really like G-technology's Mini. Old cheap computers area almost useless after less than a year. She'll get some decent software with the current Macbooks too.
Don't skimp on cars and fire extinguishers. Used computers are a terrible investment. Batteries are pretty much near dead/useless by the time you get them from the original/last owner. Macbooks come with wireless cards.
Spend money now on a decent computer with a factory warranty, included software, a new battery, and a DVD burner (for archiving purposes).
Recommendations:
Macbook 1GB RAM, Superdrive
G-technology G-drive Mini portable hard drive 160GB/$240
a spare battery for laptop
a spindle of blank DVDs
a spindle of blank CD-Rs
Will it cost you? Yes. Will it be worth it? Long term... yes.
if she's near an Apple Store, $99 for 1-on-1 training is a fantastic deal.