You can simply take a snapshot of your droplet and create a new droplet off of that snapshot. This way both droplets will have the exact same contents.

You can then modify them as you wish (remove wordpress from the first droplet, remove everything other than wordpress from the second droplet, etc.).

I want to instal wordpress with Nginx, so I can make a new installation with Nginx, varnish and transfer the content through command line or something

Then why transfer everything to a new droplet? Why not simply uninstall Apache & then install & configure Nginx?

Now to think about it, thats a better way to do it... will create a new droplet from backup and then will test on it