I am confused about which of the Sonoff Zigbee USB P or E I should chose from and which one of the two is better to work with the IKEA TRÅDFRI. I have read lots of things about the two Sonoff. -E is the most modern but may require more maintenance in therm of firmware flashing. Unless mistaken the -E version does not support Z2MQTT. I wanted to know if they both work with the IKEA TRÅDFRI and your recommendations. HA is installed in a Raspberry Pi3. Thanks.
All you need to check is compatibility between the Sonoff and ZHA/Z2M, then between the Tradfri and ZHA/Z2M.
It is the zigbee software which handles compatibility between devices, not the hardware itself.
Side note - your Pi3 might not have enough RAM to run a zigbee integration on top of your existing install. Keep an eye out on the RAM consumption just in case you run into issues.
As far as I understand both version P and E should work with the Ikea Tradfri and Sonoff. I would like to stay on the safe side with the P (although it looks like the E version is more stable now and can be flashed easly). In both cases I need to flash as router the new Sonoff prior integration into HA. Correct? I will check about the RPi3 ram. Thanks for the advice.
No, not correct.
In the zigbee world, router means any device which has the ability to pass on zigbee messages and extend the mesh. You can have as many of these devices on your network
The Sonoff is a coordinator, and this is the central brain of your network. You can only have a single coordinator for every zigbee network.
Not sure about Sonoff specifically, but it should come preloaded with coordinator firmware out of the box, and you don’t need to flash anything.
I suggest you read up on zigbee so that you familiarise yourself with how it works.
There’s a few topics in the Home Assistant cookbook and the community guides sections. I would start with this one, then continue with the linked topics at the bottom of the first post.