The only reason to go as far as making an isolated system with a pirated is if you plan to give it to someone that does not understand computers that well but wants MS office (whether it is because it is what they know*, what they think they need or something I would write off as nonsense for myself but still has some practical value in the real world) and at the same time don't want any comebacks at all in the future if it breaks. If you are doing a VM you can surely stuff the documents on a network share and then refresh to a known point every VM boot, or have a working clone sitting around should it become necessary.
*this ignores that radical shifts MS office goes through every so often -- ribbon is still not the way forward.
I don't know if future MS updates will break MS office. I normally see it for mainline windows where they sneak in a pointless update to take on piracy but can see office in the future. On said comebacks are these likely to include jumping on whatever VNC you care for at the time, loading whatever the current equivalent of the Ratiborus tools is and going from there?
In my experience those that want everything for nothing and will go here will also not care to pay you and run you ragged given half the chance. Fire them as clients. Makes life so much nicer.
As others said though libreoffice and maybe some of the online ones will do for what most people want (even given the "everybody only uses 10% of the features, just a different 10%" thing that people pondering such things pushed). I would possibly even go further and say if those don't do what you want then you need proper software -- if Libreoffice calc (their spreadsheet offering) is not doing what you want then you should probably have been using a database several steps before that point, if you need to do proper layout and such then some big boy desktop publishing will need to be involved (even MS separates publisher out a bit, whether you go into (la)tex or
https://www.scribus.net/ will probably depend upon your needs with the latter being more for poster/leaflet design, I mentioned latex a minute ago (though I am a lazy bastard so tend to opt for
https://www.lyx.org/ ) and that is what most big reports, scientific journals and all that jazz operate with and for good reason.