I used a ti-84 plus through undergrad and that was fine.
If you like being able to calculate on your phone or computer, you can also try Wabbitemu http://wabbitemu.org/ which emulates a graphing calculator and is free.
I am really surprised noone mentioned http://wabbitemu.org/
It allows you to emulate the TI-84 software on your computer for free, if you google around for a ROM file (I think it even may download it for you on the first run if I remember correctly)
If you have an Android phone you can even use it as a mobile app
A graphing calculator is used to complete most problems in stat, so a graphing calculator is very important. If you're lucky your teacher may have a class set of graphing calculators that can be borrowed.
There is a free alternative to a graphing calculator that can be used on a computer or android phone (just don't expect to be able to use this on any tests): http://wabbitemu.org/
If you've got an android phone you can get wabbitemu, a ti-84 emulator.
If you don't want specifically a graphing calculator, there's always wolfram|alpha for quick mathematical formulas.
Ask your cousin about wabbitemu. http://wabbitemu.org/ I know it can't replace a graphing calculator for testing purposes, she'll still need to have some of those on hand but that program basically can put a TI calculator on a PC, Mac or android so more people can access one. Also feel free to PM me with additional questions if needed.