Thanks already for the many answers!
I think we will first step away from the idea for the coming years and continue to save money. Basically, it’s not like we absolutely have to build. My girlfriend already owns this one house and will eventually (hopefully not for a long time) inherit another one. Same with me, the inheriting part. The idea came more from the fact that I said I don’t want to throw money out the window (= rent) anymore. Hence also the inquiry here, to come back down to earth.
About the house my girlfriend owns. She will not sell it because it has, in a way, a sentimental value for her. The income easily covers the repairs; she just doesn’t want to use the income for other things. So more a matter of not wanting to rather than not being able to, because the expenses for repairs would be that high. And yes, she keeps the place in shape, always doing repairs, and soon the roof will be completely redone. The property is huge and half of it is not even used (--> overgrown meadow), but it’s also extremely unattractive in terms of location.
The electric car will even be a Tesla (Model 3), but you can drive that forever.
Thanks again to you all!