Why do you need a VM? I'm still running 2k3 office natively on Windows 10, no problems at all.

Running 2k3 office outside of a container, vm or otherwise, is not recommended as it hasn't seen security patches in about a decade. Where you see no problems i can only think "Eeeks".

What sort of security issues do you expect Office 2003 to have that will affect you nowadays?

Loads. Simply loads. Office Macros execute by default in 2003 right?

Hopefully no one is using the outlook from then as their mail client at least...

Quite a few of what you can see in this list [1].

[1] https://www.cvedetails.com/product/320/Microsoft-Office.html...

Yes but which of those apply specifically to Office 2003, on Windows 10 and will be exploited today in 2019?

Because I don't want to run windows as my main OS?

Why not native LibreOffice? Does MS Office from 10 years ago offer something that LibreOffice doesn't?

I believe all (most all?) of the MSOffice VBA automation doesn't work in LibreOffice.

Sadly yes. I use LibreOffice where possible but sometimes perfect bug compatibility is needed.