Hi,
Use Polyisocyanurate insulation. This is the type that usually comes with alum foil layers on one or both sides. The stuff we get here is kind of light brown in color and has very fine grain. Around here, if you ask a lumber yard if they have Polyisocyanurate insulation they say "huh", but if you look closely at the insualtion panels it says right on them that they are Polyisocyanurate. This is what is used in most commercial solar collectors, and should work fine. As you say foil on the inside is good -- reflects heat back to absorber. Foil on the outside is also good -- it has a low emissivity, so it does not radiate heat to the outside well (which is good). For these reflective layers to work, they need to be facing an airspace of at least half an inch -- ie placing the reflective foil right against the absorber will negate any benefit from the foil.
Don't use the polystyrene insulation board (this is the pink or blue or white beaded stuff) -- it will not hold up to the temperatures.
Gary