replacing door leading into sunroom
If your sun room is really a three-season room, you need an insulated door between it and your heated space as part of what's known as the "building envelope". Interior doors not only don't insulate as well, but they don't offer the seals that prevent airflow (which is actually a greater source of heat loss). 

If your sun room stays within say 20 degrees of the heated space year round, you can probably go ahead and use interior doors, but I'd still consider adding bristle-type air gaskets.