Lots of good points.
Additional comments:
If you have a good verandah (good eaves generally) then external walls may be less of an issue.
As far as taking parts of the roof off, that depends on the roof construction material. Tiles - fairly easy. Corrugated iron - not as easy. In either case you want to make damned sure that the roof is still watertight after the job has been done.
So … the cost has to be assessed based on an inspection of the details of the specific property.
Properly designed energy efficient homes will have some helpful features (good roof pitch, good eaves/verandah) but some unhelpful features (wall insulation).
