Great, this is very helpful. Thank you very much for your detailed response!
Would still have to discuss how many bus lines I will need. But seems like there are tutorials how to set up the knx IP router in case I need to.
Regarding the SPS devices, I would not be worried so much if automated functions stop working if my openHAB server is down. However, I would always want to be able to operate the heating via thermostats and to turn on some lights in such cases. Is that what you mean with some low level functions?
I do have a strong preference for knx over loxone. However, apart from the price the advantage with Loxone could be that I would get a fully running system before I start tinkering with openHAB for the first time. And my wife wouldn’t appreciate it too much if we’re moving in and the lights don’t work for the first few weeks. 
Lastly, I would be interested to understand exactly why you recommend an ETS license. My understanding is that I can set up the system only by using openHAB. Or do I first need to configure sensors and actuators within ETS before I start working with openHAB? It just seems that I wouldn’t need ETS once the system is running so it was one of the first items I was hoping to save money on.
Thanks again for any further feedback!
Jens