Suggestions on which NUC/Optiplex to buy

[TheDreadPirate]

Any optiplex or NUC with 8th gen Intel CPUs and newer would meet your needs for transcoding. If your content is HDR, an iGPU may not be able to handle 3 simultaneous streams since tone mapping is significantly more demanding than plain transcoding.

8th gen Intel+
4GB of RAM minimum, preferably 8GB

If you get an optiplex and have some budget left over, consider adding an Intel Arc A310. Super cheap, extremely capable. Would definitely be able to handle 3 HDR tone maps. Not sure what the availability is in the EU. But they can be had for less than US$100 here. Ignore this if your content is definitely not HDR.

BTW, I removed your other thread in troubleshooting since you made this one. I was about to move it here anyway.