Converting from traditional aluminum siding to smooth skins?

I would dissuade against fiberglass batt insulation and use rigid instead.

Stay away from the commonly referred block foam (styrofoam) which is the cheap expanded polystyrene (EPS) and use the blue or pink extruded polystyrene foam (XPS) instead. It has a better R-value and is reasonbly priced. If you want a better R then go with the polyiso (polyisocyanurate.) It is more costly but provides the best R-value for limited space.