Raj
TreeHouser
They lay steel over shingles here, no insurance or building code issues. If it were my own I'd strip down the old.....if I had the time.......
Where you insulate depends on what type of ceiling you have mostly. Generally you want air to circulate under the roof (in the attic) to prevent condensation in and over-heating of the roof. Insulation is usually (but not always) against the ceiling, and air against the roof.
If the ceiling is the roof, then you would insulate the roof.