Exterior walls of the basement not accessible - Is this problematic?

If you don’t mean just the partner but also the house in question, then I wouldn’t worry. Super well maintained is typically not the impression that a disguised dump would leave.

But that is not good advice....from my experience...
That’s exactly how I ended up buying a damp house....it looked so well maintained, only 10 years old, all the best materials, built for oneself, etc etc....and it was damp nevertheless....

Therefore: always buy with a building expert and this person should not be someone from the internet but a publicly appointed and sworn expert ... e.g. to be found in lists of the Chamber of Industry and Commerce ....