I am thinking of a simple project.
Ok then:-
The aim is to control multiple stepper motors, small water pumps, vacuum pump only one and several web cams and a lot of temperature and humidity sensors.
So why did you describe this complex project rather than the simple one you were thinking of?
What do you mean by control a Web Cam? The arduino is not fast enough to do anything with a video output.
Do these motors need to move at the same time? If so that increases the complexity.
or is it too expensive for a project like this?
No, but it is a lot more complex than you think.