Aluminum you'll for sure have to insulate -- not only would it be quite cold, more problematically you will have TERRIBLE condensation and humidity problems if you don't insulate. If you look around you should be able to scrounge cheap or free insulation material and something thin to cover it, so it needn't necessarily be expensive. You will probably also want to either cover over, or at least tape bubblewrap onto, most or all the windows for wintertime for the same reason, since they are such a large expanse.
Do these windows open? If they do, heat may not be too much of a problem in summer. If they don't open, make sure you have sufficient ventilation elsewhere (probably need more for this sort of building than for some others) and you could whitewash them as your husband suggests or cover with shadecloth or something like that. (Shadecloth is great, it cuts most of the sunlight but if tightly strung won't flap in the wind like tarps do).
Whether to have a raised or dirt floor just depends how you feel like constructing it, really. There are pros and cons of each. (Although if it'll be in an area that gets snowmelt runoff or puddles after t'storms, I'd for sure do an elevated floor). Construction is different for the 2 types and predator-proofing methods differ, but I'm not convinced that one is massively superior to the other for most situations.
Good luck, have fun,
Pat
