Hello,
Both Office 2008 and 2011 do work in El Capitan. However, Microsoft dropped support for Office 2008 back in 2013, and it has a few quirks like slow launch/quit of the apps ever since Yosemite. I know of many folks that run Office 2011 just fine in El Capitan. If Microsoft is consistent, they will probably officially drop support for 2011 within a year. Office 2016/365 is fully compatible and supported.
Both Office 2011 and 2016 are backwards-compatible with Office 2008 documents, so you'll be safe there. It is really up to you on which version to purchase,...just remember that 2011 support will eventually be dropped.
Personally, I don't use any Microsoft software any longer, other than Microsoft Remote Desktop, which is required to access some things I do for work. I use Apple Keynote/Numbers/Pages and Google Docs/Sheets/Slides for anything Office-related.
Hope that helps,
C