RE: Stocking up on balsa
If you're looking for some general "good to have" and not a complete stock for scratch building I'd go something like this:
-Medium sized triangle stock for reinforcing firewalls and landing gear plates
-Few balsa sheets in diffrent thickness
-A sheet of lite ply
-Few diffrent sized hard wood sticks for general reinforcing/repairs.
-Few balsa sticks
-Perhaps one or two round dowels...