In some cases, you'll learn more from crappy hardware than you will from lab-grade gear. This is probably one of those cases. This build will require thoughtful attention and debugging/optimization on the student's part in ways that the Thor Labs kit might not.

I mean, it's practically the most basic optical experiment that you can perform. Nobody needs to pay $3K to learn how an interferometer works. It's not a MoT or something exotic like that, it's a beam splitter and a couple of mirrors.

Put another way, it's the difference between building a Heathkit and putting a bunch of parts together that you salvaged from other stuff, for those who are old enough to grasp that analogy.