Thanks to the great tip in this thread,I am writing this from a Ugoos UT3S, I got for 36€. So, now I would like to give something back to the community and write about my experiences.
I already flashed it to the latest available release from Ugoos (Xubuntu Vivid) in a linux only configuration. For this I used a self-made OTG cable, because contrary to what was advertised, that was not included. I flashed from another linux machine using Rockchip upgrade_tool and rkflashkit. Windows AndroidTool on Win7 didn't work at all. upgrade_tool somehow only worked for the bootloader and the parameters. Especially flashing the misc image didn't work so that the device kept rebooting back into loader-mode. Fortunately, rkflashkit solved that problem.
After some toying around with it, I rebooted and was shocked to find that the display remained black. After lots of frustrating experiments (including a complete reflash) and googling I learned that the display is in power save mode and I have to do Ctrl+Alt+F6 Ctrl+Alt+F7 after every reboot. It seems something in the combination of the kernel drivers and my LG display doesn't work too well together. But thats acceptable for now.
I have now installed some more software, including mackodi (http://mac-l1.com/) , which gives me all the entertainment I can are for. My current impression is that this tiny box could well become the desktop machine for my day-to-day use.
Only some things are bothering me:
I'm stuck at Xubuntu 15 with no more support and updates. Especially being stuck with FireFox 44 gives me heavy security concerns
Bluetooth seems to be broken. It still worked with the factory installation of Xubuntu 14.04. But maybe the installation of mackodi broke it, I not sure when it stopped working.
Ugoos settings for the LED doesn't work either anymore
I don't yet have an AV cable for proper audio. Kodi doesn't seem to work with HDMI audio and the normal three-sleved 3.5mm-jacks give me only one channel
So, to conclude: getting a more recent, more fully featured linux on this machine would be really worth it. If anybody has any suggestions for things I could try to improve, that are in the range of my capabilities, I would be happy to try them out.