I'd recommend keeping the Fire tablets with their stock OS if the goal is to use them for displaying dashboards.
All the tablets I flashed and put Google Play on end up getting really sluggish after running for a few days. Whereas the tablets that were unpackaged, turned on, and then immediately had Fully Kiosk Browser installed (and no other tweaks or sign-ins) run nice and fast. 
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Similarly, I'd recommend at least a Fire HD 8. The bigger Fire tablet devices have better processors and performance. The 7 is maybe fine for basic dashboards, but they start to get sluggish with media tiles or complex dashboards.
