If you subscribe to Office 365 you should be entitled to the latest version. This means that if you do depend on Microsoft Office you should upgrade to Office 2016, which starts at a one-time payment of $149.99 or $6.99/month for their monthly plan before you upgrade to High Sierra.
However, Microsoft is ending support for Office 2011 in October and as such is not updating it for compatibility with High Sierra. This is largely a refinement update, with lots of exciting new technologies under the hood that should help your computer run more reliably and more securely. Later this month Apple is releasing a free upgrade for their Macintosh operating system, macOS 10.13 High Sierra.
