I've tried to get Canon 5D/6D but they are becoming pretty sought after (because of the availability of lenses). Another problem is when they are sold cheap they have huge shutter counts often way after their spec lifetime. Not sure about Nikon maybe there are some cheaps. You can probably get 6D mark I with pretty high shutter count for around 200e.
Best usable 5D mark III deals i could find were around 450eur (thats camera from 2012). I ended up getting Lumix S5 (mark 1, from 2020) for like 500eur that is very different beast of a camera while having L-mount which is becoming only "open" camera mount (third party lenses are being "disallowed" by most manufacturers now).
Compared to moderns systems the main difference is the autofocus and video capabilities. Modern mirrorless have cosmically better tracking, eye detect etc.
For instance, human eyes can't perceive the difference between a 12MP and a 50MP image printed in a poster format from a typical 1.5-2meters viewing distance and 8MP is usually good enough for most large prints.
So I would advise choosing a second hand model taking shutter count, general state, lenses quality, autofocus speed and image stabilisation efficiency as more prioritary parameters than sensor pixel count.
That means a camera APS-C or micro four thirds sensor might suit better to someone who is new to photography.