To get input data from the mouse you got either InputPoint class or PickHelper. It depends what you are looking for. InputPoint gives you inferences is is similar to what for instance the Line tool uses. PickHelper is more for being abel to select Entities. Either way, You want to oolk into the Tool class.
As for units which jolran mentioned, I got an overview of how to deal with units in SU here: http://www.thomthom.net/thoughts/2012/08/dealing-with-units-in-sketchup/ You basically should not do so much work with it. Keep everything in inches, the internal units, preferable in the Length class and let SU do the work of converting back and forth between internal units and model units.