I am going over the manual of the modelling section: http://www.mujoco.org/book/modeling.html and it seems all quite complicated and it seems that it is easy to make mistakes in the process. Is there any way of generating the MJFC models with the GUI interface or do we actually need to create everything by hand?
It might be overwhelming at first sight, but is actually quite nicely structured. I certainly wouldn't bother with any gui. If you insist on a GUI, the only thing I am aware of is SolidWorks. It has a Urdf Exporter, which you can the import into mujoco. Unless you are already mastering SolidWorks, I guess this is a pretty inefficient and even more expensive way to get your robots up and running.