Can these doors or frames be painted? Or replaced?

I think your door issue has already been addressed. I had recommended self-adhesive film. Otherwise, I would have left the basement as it is for now.

can you recommend a specific film? I would like to try it on the door frames in our basement.

Regarding the question, yes, from the photo it looks more like artificial wood than real wood veneer. But you can tell if you lightly sand it. Let's assume it's plastic. Then please clean it properly with soapy water, degrease with spirit. Light sanding, around 120 grit or a bit finer, dust off well, apply alkyd resin varnish of your choice, preferably semi-matte, high gloss doesn't forgive mistakes. Use a mohair roller, take the one made of short-haired teddy plush, roll over. If it’s white, one coat should be enough. Why alkyd resin varnish? Because you can thin it slightly with universal thinner, then it levels out perfectly and even as a layman you get a nice smooth surface. Acrylic water-based paints do harden and don't emit fumes, but they level poorly. The result is often not so smooth, like orange peel for example.

Alternative, you only paint the frame and take the door leaf to the auto painter and ask him to spray it quickly. It probably costs around 200,- but of course gives a result better than new.

Thank you very much! Now another question. Is it possible to install a glass door (a hinged door and not a wooden door with a glass panel) into a standard wooden frame? I would install reinforcements behind the hinges. Is it possible to combine this with a standard wooden frame according to DIN standards?

One more quick question... does anyone of you know HORI or has experience with it? Are these comparable in quality to Prüm or Herholz?

Thanks and regards

can you recommend a specific slide? I would like to test it on the stringers in our basement.

My husband unfortunately forgot the company. But it was not completely smooth, rather white with a kind of wood texture.