unboggling, with regards to upgrades, one is usually better off to buy them upfront than wait until later, otherwise you're paying for a hard drive or RAM that you'll need to remove and dispose of. In my experience "upgradability" isn't very high on the list of requirements for a laptop; I'd rather just exchange it for a newer model.
I guess my main point though, is more psychological than technological. I've worked as a Mac tech for more than 20 years, and have observed a lot of intersting (and sometimes bizarre) behavior in customers, friends, and relatives. From the tech-head who's obsessed with having the latest and greatest gear even though he (and it's usually a guy) has no actual need for it, to the person who's still running six-year-old versions of their software even though it's horribly broken and there are free updates available, to the one who never buys a computer because there's always something new and better coming up in a few months from now, and they want to wait... there's a very interesting field called "behavioral economics" that studies all the irrational yet predictable tendencies that humans have with regards to ideas of money, value, and purchases. The book "The Paradox of Choice" by Barry Schwartz is a great introduction to it.
It seems to be human nature that people develop a personal relationship with their computer. And one of the most common things I've observed is people hanging on to a computer for too long, even when it doesn't meet their needs, and they'd be better off with something else. You don't dump a boyfriend/girlfriend or a pet, just on a whim - you've made some kind of committment to a relationship with them. Sometimes I think people extend that kind of feeling to their computers. I've had to learn not to do that myself...
Anyway, none of this is directly to do with your niece, just some general observations about the "which computer to buy?" question. Good luck to her with her studies, I'm sure she'll enjoy her Mac!