The best way to meet your low power requirement, which you listed as the most important, is by having flash storage. The other half of this is that used hardware is going to be worse at power management than an SBC too. It may be worth doing the math to see how long the power draw difference will take to even out the cost of using flash storage instead of magnetic storage.
The other part of this is that you could just grab a 2.5" hard drive and split the difference on price/GB. https://uk.pcpartpicker.com/product/xCKhP6/seagate-barracuda-5tb-25-5400rpm-internal-hard-drive-st5000lm000 start with two, and if you run out of space, or obtain more money, add two more from a different brand.
It really depends on the model. Best to pay attention to it like the previous comment mentioned.