Windows Defender and Anti Virus is just fine, and all built in.
I like Libreoffice as a free office suite, and not had any file compatibility issues with PC version MS Office. I imagine if you are only doing simple financial type spreadsheets it’d be portable to/from Mac, but I’ve never done that personally.
You can buy multi-user versions of Office 365, but we installed the free first month on my daughters Laptop, then logged into her 6th form supplied account on that computer. It’s worked fine ever since, so one account across multiple PC’s seems OK, as long as you don’t mind sharing the cloud storage with the other user. I think.