I will do a final recap.
I have installed Plex Media Server on the Raspberry Pi3, wifi and bluetooth are disabled. I also installed Deluge with the scheduler plug-in(downloads happen in the middle of the night). Samba is installed to map my Netgear ReadyNAS. Plex data is stored on the sd card (Samsung Evo+ 64gb) but the media files are on the NAS. Deluge downloads directy to the NAS.
On the clients I installed OpenPHT and defined that the transcoding would be done by the client. Currently the system is able to deliver 3 simultaneous streams without any hiccups, even the famous x265 videos stream without troubles.
Overall I’m very happy with the setup and I’m impressed by how well the raspberry performs (I was very sceptical about the overall performance).