How do I compute the surface & volume of a large amount of STL files?
You could use the Python console of FreeCAD. Issuing the following commands

FreeCAD.ActiveDocument.ActiveObject.Shape.Area
FreeCAD.ActiveDocument.ActiveObject.Shape.Volume


will output the area and the volume of the object, respectively.

I don't know how to access the console output from outside FreeCAD, but there is an option in FreeCAD called Redirect internal Python output to report view which I think is what you are looking for.

You might want to look into the Scripting documentation of FreeCAD for further instructions.