ᐅ Video door entry system for wireless retrofit installation

Hello everyone,
I saw the other thread about video systems, but that one is about “proper” systems. It doesn’t quite fit my situation, so here is a separate post:
The house belongs to my in-laws, it has two floors and the interior was only partially renovated recently. The house also has an old doorbell system, but I’d rather not touch it. Unfortunately, the front door is made of glass, and the older folks would like to be able to see who’s at the door from the living room via video.
I’m not supposed to install any cables, and app control or similar features are not necessary. The video quality should just be good. The distance between the outdoor unit and the indoor unit is about 6 meters (20 feet).
Now I’m looking for something that delivers reasonably good video, works via WiFi or wireless connection, with an outdoor battery that lasts a decent time and gives an early warning when it needs charging. The budget is under 500 euros.
Do you have any suggestions?