Regarding Microsoft Office, be it 365 / 2019 / 2016, or even Windows, one can buy cheap licences for a bargain on eBay. Focus on european resellers at one of the european domains (like eBay UK).
Under the right circumstances, it appears lawful to sell & purchase second hand digital licensed software as guided by the EU justice court...
Likely, this ruling shall extend to the trading of trading platform or addons’ licences, even when the publisher discourages you against from a special licence agreement
If 365 is chosen, in practice, just do not use the cloud storage like OneDrive and SharePoint, that is avoid backing up and sharing files online (in the rare event the access would be taken over by desactivation or the general administrator changing the pw). Instead, download and use the Office suite locally from the official links and you should do perfectly fine.
Some MS options’ tweaks