I don't see why you couldn't get a N900 and just not use the phone feature. Unless you live somewhere where you have to buy a phone plan to buy a phone (read: The U.S., the only country I know where you can't buy all phones separately without a subscription from any phone stores. And even there you might be able to import one without a subscription.).
I've run unstable for years without any major problems. But then I know how to fix problems I encounter, and I don't run gnome or kde =P
A gram of fat is 9 kcal and a gram of protein is 4 kcal for a person. Not sure if there is anything else in the human body that would store energy, the carbohydate amount is small enough that its neglegible.
So a person weighing 75 kilos would have 155 250 kcal in his body. I imagine that when burning that however a lot of the energy would be lost to evaporating the water in the human body.