Well, first off; you can convert whatever format you find with Calibre very easily. ePub is the most widely available and everything but the Kindles will support that (and Calibre can covert to Mobi which Kindles will read).
Secondly, I really prefer Android readers (like Onyx, Icarus, Boyue, etc.) because they are truly open format. I use Google Play Reader on all my devices (phone, tablet, Onyx Note, and even my Sony TV) to sync the same content and pick up where I left off reading on the other devices.
I use Epubor's Ultimate app to convert my Amazon books to ePub and then I stick them in my Calibre library before uploading whatever I want to read to my Google Play Books library for free (along with PDFs and ePubs that I get for free from other places).
Protip: use the Calibre "Edit Book" option to "Run Check Now" and look for/fix ePub errors if you are going to upload to Google Play.
If you can wait a couple of months, there might be a "sale" on Black Friday that drops to your budget, but I think that's a longshot.
Even this refurbished Nook Glowlight Plus is just over your budget.
Personally, I'd avoid this Kindle Fire 7 both because it's LED and not eink and because it has "special offers" (ads on the lock screen). But it's at least in your budget.
I like the Kindle Paperwhite- same as the Nook Glowlight, but you can adjust the brightness of the light, which I couldn't do with my Nook. I love the interface and borrow a lot of eBooks from the library, which I find easier to do with my Kindle than my Nook. https://www.amazon.com/Amazon-Kindle-Paperwhite-6-Inch-4GB-eReader/dp/B00OQVZDJM/ref=sr_1_1?ie=UTF8&qid=1486490196&sr=8-1&keywords=kindle+paperwhite Unfortunately, it's only 6" but you can adjust text size to smaller or larger. I have not seen any eReaders that use eInk to be much larger than this...
It's a little smaller than what you are looking for, but it's really easy on the eyes, you turn on the glow light as you need it, and you can adjust the brightness.