Just my humble 2ยข, but have you tried FreeNAS? I know that most of the features you are looking for are available, they just require some configuration. Of everything I saw mentioned, the only one I haven't worked with myself was the UPS support (but the site says it is there). I personally am using this right now, although I am getting ready to repurpose the machine to do something slightly bigger than just act as a NAS. I am not sure if it will run on an ARM processor though...
Michael Haworth
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