Which version to use where?

Hi,

I've succeeded in completely losing myself in the maze of available & supported development platforms for Office, I hope you can help me...

What I want is this:

I would like to develop a custom pane for Word, Outlook & PowerPoint, and I want it to be available on O2013 & 2016, and also on Office On-Line, iPad, and the soon-to-be supported Mac.

Can I use the current App model for Office 2013? Will it work as-is on Office 2016? Or maybe I should use some newly released model specific for Office 2016?

If the current dev tools (the ones that come with VS2015) are not enough - which ones should I use?

Thanks!