Hey drake,
I’m no pro myself, but here are my suggestions to your questions:
1 - What would be a good/safe overclocking for my setup? or maybe I shouldn’t overclock at all?
I’d say stress test the Raspberry Pi 3 first without any kind of overclocking to see if it will be able to handle the load. I’m not quite sure if you plan to go headless or not (no monitor attached to the video output) but you could watch how much load is on the processor by using the htop command either by using the terminal directly from the pi or an SSH connection through your network. If the load is sufficient by your standards and nothing lags, I wouldn’t overclock in order to give that extra protection and unneeded wear and tear on the pi.
If you are willing to overclock you system, it appears that there is currently no support on overclocking a pi 3 as of yet (meaning it could be risky). It seems like it is still possible to do based on this blog post if your up for the task: http://www.jackenhack.com/raspberry-pi-3-overclocking/.
2 - Should I overclock the sd card reader speeds ? The plexserver metadata is on the sd card, the files are on the NAS.
You could attempt to overclock the sd card bus speed, but you risk the chance of corrupting data on the sd card if the card is too slow to begin with. Take a look at the forum post here to read a little more about this: RPi | Double SDcard IO (Read/Write speeds)
3 - It’s there some services/stuff I can disable to get better performance? I don’t need wifi nor bluetooth. I aready setted 16Mb to GPU.
There should be some menus within the dietpi-config command that will allow you to disable a few peripherals (wifi, bluetooth, hdmi, picam). You might not obtain all that much performance turning these off, but it would help to cut back power consumption. If I remember correctly, most of these are disabled to begin with too. Background processes are pretty minimal from the start, so most of the processing power will be going between your plex media server and torrent client.
4 - What torrent software you recommed me ?
I haven’t tried any of the torrent clients myself (I still use utorrent on my windows machine for now) but I would say try each one found in the dietpi-software command. Depending on how you plan to run the torrent client, one might stand out to be the better pick. Dietpi is optimized for the following four clients:
Transmission https://dietpi.com/forum/t/dietpi-software-details-for-all-installation-options/22/1
Deluge DietPi Community Forum - Welcome to the DietPi OS Community Forum
qBitTorrent DietPi i2c problem
rTorrent https://dietpi.com/forum/t/dietpi-software-details-for-all-installation-options/22/70
Hope this helps, and good luck with your build! Let us know how it turns out too.