After doing some more in-depth calculations, I don't think it will be possible to run a backhoe in the required space behind our home. If it is to be dug there, it would have to be by hand or small tiller; that is not preferrable.
My other (and probably better option) would be to build it into the crawl space and install a trap door down from the kitchen. The difficulty here is that I am 6'5" and my crawl space at that point is 5'10". I really am weary of digging below the vapor barrier for fear of disrupting the foundation, though I am sure it could be done safely with a contractor. This wouldn't afford me as much of the ground temperature advantage, but would greatly reduce upfront costs and possibly better improve the home value.
I will update with future findings. I plan on undergoing construction (whereever that is) by February, so remain patient and feel free to interject and comment. I am also open to cool additional ideas (ie incorporating a humidor?)