Around here there are a few contractors that specialize in this type of thing. I think this is definately a big project, in scope and money, and best left to a professional. Just make sure that if you are going to go to that expense, make the new basement walls at least 8' (hopefully 9' or 10') and have it pre-plumbed for a bathroom.

Thanks! Unfortunetly my existing cellar room only has 7 foot ceilings, going deeper than that in the crawl space may not be real practical, and the existing foundation walls only go down a few feet, so as the crawl space was dug out something solid would have to be put under them to support the house.