Siding removal

Since you have no idea on how it was fastened or how hard it will be to removed I'd do it by the hour to be safe.
But I also would have just gone over what's already there unless it's water damaged or I was planing on insulating or rewiring while it was open.

insulation under retro siding

If I were to go over existing siding, I would add some insulation between old and new. I've heard about the foil bubble wrap type but also tend towards the plastifab (solid type) insulation. Suggestions? As to wiring, internal insulation etc. the ball is in the owners court at this time.