To answer your questions:

1. Yes, it is possibe to use it as a seedbox and to install rtorrent + rutorrent on it.
2. You can use Plex on it.
3. You can quickly deploy an OS from the Hetzner control panel. You can choose different OS's. You can choose from CentOS, Debian, OpenSUSE and Ubuntu. I don't know about Ubuntu MATE. But because it is a dedicated server you can deploy whatever OS you like.
4. You have scripts for some stuff, but general Linux (command line, etc) knowledge is recommended. And of course there are search engines.
5. Ubuntu is a good choose, however I see more seedbox providers running Debian on their boxes (for e.g. Feral Hosting).

When you go with Hetzner, don't use public trackers. Hetzner is based in Germany and the German copyright law is quite strict.
But when you just use private trackers, you're ok.

If you need some more info, just let me know.