As a licensed builder I can say the best idea is going to a 4x4 site and asking advice about your house.
The amount of variables that need to be answered before you can make a decision on this prove this an impossible question to answer without a visit to the house to diagnose the conditions. And no, a crawl space does not necessarily need to be conditioned. Do you want to add that much area to your heat bill?
Call someone in to look at this before you just throw money at it.