As far as I can tell, the oldest version of MS Office that's compatible with W10 is Office 2010. That's according to the official MS page. They claim that Office 2007 works, but is not supported or tested.
As you can tell, MS is pushing their subscription model. They prefer that you rent MS Office as opposed to buying it outright. However, you can still buy the Office 2019 program, its just not advertised. Of course, you won't get some of the perks of Office 365, like the 1 Tb OneDrive storage and a few other things, but they're not necessarily important.
As far as those cheap keys go, its a bit of a grey area. Sometimes, businesses buy what's known as a "Volume license". This allows companies to use a single key on all of their computers. It is sold to that company with the condition that it can only be used by that specific company. Sometimes these keys are "leaked" and/or stolen so people resell them. As far as I can tell, it isn't illegal to use them, but it is against Microsoft's terms of service so they could blacklist those keys, making them useless.
As the other guys mentioned, you can use free software like OpenOffice or LibreOffice. These are alternatives to MS Office, and are completely free. They're mostly compatible with MS Office documents (though I have sometimes run into some slight compatibility problems in some documents.)