You can usually score Microsoft Office cheap with student discounts or by hitting up sites that sell software licenses for less. Or just use free stuff like Google Docs or LibreOffice those are good enough for basic stuff, even if they don’t have all the fancy features.

I grabbed Microsoft Office from a site, and got a sweet deal on a lifetime license. It's legit and way cheaper than usual. I use it for school and work, and it's been solid. Check out the Microsoft Office Professional Plus 2021 version there; it’s affordable and got everything you need!

Start by asking at your current university. A lot of them have a deal with Microsoft that allows a purchase at discounted prices.

Missing a cab file does seem to indicate a problem with the installation media. The valuable bit is the key !

edit: ebay lists a few sellers offering the cd's cheaply.