Which office do I need for my MacOS HighSierra

If you mean an app like Microsoft Office, then you don't since Microsoft no longer supports macOS 10.13 since MS took the activation servers offline for older MS Office products.

If you previously "purchased" the Apple iWorks apps Pages, Numbers, or KeyNote on macOS 10.13, then you can redownload them from the App Store, but if you never previously "purchased" them, then they are no longer available to download.

The next alternative is the free & open source office suite LibreOffice. The following link shows that v7.4.x.x is the last version compatible with macOS 10.13 High Sierra:

https://wiki.documentfoundation.org/Documentation/System_Requirements/fr

There are no 2015 model year Mac Pro models so more information would help: Identify your Mac Pro - Apple Support.

Any 2015 model year Mac can be upgraded to a later version than High Sierra though. How to download and install macOS - Apple Support describes how to upgrade Macs in general.

Which office you require for it does not seem relevant to me. You can use a Mac Pro in any convenient location.