Criteria when buying a laptop?

What should I look out for when buying a laptop? Which criteria are important besides the processor? My budget is 400 euro

You have to see which hard drive is installed.
An SSD is always best

It depends on what you want to do, but 8 Gb ram should be in there

Screen, RAM, built-in hard drive possibly, battery (if that is important to you)

The decisive question is what you value.

Performance, display, battery life, mobility, workmanship? Depending on the situation, other criteria are decisive.

I would recommend at least an Intel i5 or Ryzen 5, rather take something faster and have something from the device for longer. I would also recommend at least 8GB Ram and at least a 256GB SSD or larger, since memory is usually full quickly and Windows itself already occupies a larger part of it.

Depending on what you are looking for, there are also good used devices from Lenovo, I can highly recommend the Thinkpad series.