To renovate or not - a garage tale by cpdbfbmc

http://imgur.com/a/uAlX9 - pictures.

Our garage needs some help.

First, the doors. The doors and tracks are in rough shape. We are absolutely going to replace these, ideally with insulated doors. As for the track - is the torsion spring setup an option with the ceiling height, HVAC and drop ceiling? We plan to add an automatic opener. I believe the torsion spring is better suited for an opener?

Second, the insulation. R-13 is currently in place. To me, this is low. My plan was to put R-19 - does this sound right for a garage under the main living area? As mentioned above, we plan to put insulated garage doors in place so climate control is the end goal. Does spray insulation seem like an option? I assume not because of the drop ceiling and access to plumbing, electrical, HVAC etc.

Third, HVAC. It appears that mice chewed some of the HVAC duct. It does not seem to have penetrated the plastic inside, but just the insulated portion. Is wrapping this section with new insulation sufficient or should the entire line be replaced?

Fourth, notice where the main HVAC duct moves from the garage to the house (area where HVAC unit and water heater are located). This area is 'open' and not insulated. Is this by design? I don't see why this should be closed off for better temperature regulation. The previous owners have a sheet of cardboard covering part of it.

Thanks for your insight!