How to install drush 8 properly on Debian 8? Finding hundreds of articles and documents about how to install drush and none of it explains where exactly and how properly I got a bit lost.

I've got a Debian 8 system with a root user, NGinx and a ssh user.
Now I'd like to go with drush 8 to be ready for Drupal 8 soon.
But I don't feel like installing git on my server - is this common practice? I usually think the fewer software the better?  

So - any chance of installing drush without having git? 

Next thing that all instructions seem to forget: 
As which user do I need to install drush and composer? (I did it now as the ssh user with the result that root can't use drush).

And eventually: Which directory would be correct to have drush installed? /usr/bin/ ? With a symlink? Or directly there? 
Does the NGinx user (www-data) need to be able to use drush? (I guess yes for drush pm-update?)

Very lost here - hope someone can shed some light.
(Updated to fix links and clarify Drush 8 vs 9.)

Debian still (as of February 8th 2018) comes with Drush 5.10 which is completely unacceptable.

There is no reason to use Git to download Drush. Either 


If you want the old Drush 8 (that works for Drupal 8.3 and older, including Drupal 7) download a single PHAR file using the the straight forward instructions on http://docs.drush.org/en/8.x/install/ or
If you want the current Drush 9, install a site-local Drush (with Composer) and Drush Launcher (as a PHAR file) as described on http://docs.drush.org/en/master/install/