Security-wise, being the only house on the block with no snow on the roof might be noticed. Keeping the whole attic warm is pointless and costly

I'd probably build a complete small box or two up there, and not use the floor or roof of the attic at all..Then you only have to worry about the climate in a smaller contained space. Should eliminate most of the problems of a large area.

15 plants can be a big grow or a small grow - Realistically what size lamp(s) and what kind of yield do you need to make it worthwhile? Lets say Monthly (although you know you dont harvest monthly unless you go perpetual - right?) How many ounces/month do you need to pull?