Selecting low-power managed flash storage?
As someone who has made FLASH, I would say that you should just use a MicroSD card.  Firstly, it's really easy to get.  Secondly, you cannot probably afford the cost of dealing with suppliers for what you'll get out of it.

Most FLASH storage is relatively low power to read, but writing takes a lot more power.  My FLASH took 1nA per bit for 1uS to program at 5V.  The erasing took a lot more power because we used tunneling and had to "ramp up" the circuits.  Basically, we erased blocks and then programmed bits.  

I suspect that the power cost of FLASH is just a drop in the bucket of total power consumption.