Unfortunately, there is only one overall floor plan and no - let’s say - partial drawing of the bathroom as well as the other rooms.
or should I measure everything myself completely?
If I plan to renovate (my) bathroom, I would have already made this drawing myself. And yes, also measured it myself.
Where do you envision the new toilet? You don’t have many options.
ideally a walk-in shower
Water flows downward, not upward. If you’re lucky, the tub is sitting directly on the floor without screed, then the possibility of a level shower would be given.
Price from.. to.. depends on what you choose and who will do it.
Overall, the bathroom makes a very good impression - whether you really need to tear it out, you have to decide... I would just spruce it up with a stylish washbasin, mirror, better lighting and possibly cladding. For lighting, a high or frosted window section in the door could be useful.