Somehow I missed your other questions in there.
I've never gotten a headache from reading on my ereader. Make sure you get an e-ink ereader though, and not a glorified tablet like the Kindle Fire. E-ink light comes from above the page and shines down onto it, which is why it's much easier on your eyes; tablets (and phones) all have typical back-lighting, reading on those for hours will be harder on your eyes, many people do get headaches from that. The battery life is much better with e-ink screens than tablets, a couple of weeks with e-ink vs a day with a tablet. It's one of the reasons I'd never use my phone for reading, uses way too much battery up. And I find it extremely annoying to read on a such a tiny screen. You can read outdoors in bright sunlight on an e-ink ereader, can't do that on a tablet or phone.
I've never had any ereader (or even a tablet or computer component for that matter) emit any plastic or chemical odor at all, I'd say they are definitely odorless.
Kobo also has models that let you change the hue of the front light to warmer tones. I have crappy vision, old eyes make it even worse, but my preference is a whiter or bluer light, it gives more contrast and I just dislike the warmer colors for some reason. That even goes for using "warm white" light bulbs in the house, I just don't like the color. But having the options to change to your preferred color is always a big plus.
How long they last can't be measure by how many books you read. I have my Sony readers from 2010 that are still working just fine, even the batteries are holding charges. The reader I've been using since January 2016 is the Kobo Glo HD, and it's still perfect. I read every day, and the reader goes with me every time I leave the house. When I'm out on long walks, I love to find a cozy place to sit and read for awhile. Most e-ink readers have a glass layer, so you can break that substrate layer by any twisting or flexing of the reader. And it's best to keep it protected in a case. The sleep cases are just like opening a book, taking you right back to the page you left off at. But I've never once broken an ereader. Just be mindful if you pack it into a large purse or backpack to not have anything pressing against it oddly that could flex it or put pressure on the screen. I believe that Kobo now has an ereader where the substrate layer is plastic and not glass, so that would be very durable.
Downloading from email... I think you can send ebooks to your Kindle, but probably never a library book because of DRM. Many users use a program called "Calibre" to keep track of their library, and transfer books to their reader with a USB cable. If you buy directly from Amazon or Kobo, the books will be downloaded directly to your Kindle or Kobo reader. You won't have to email it to get on your reader. The other advantage to Calibre is if you ever lose or break your ereader, you have your books backed up and can just transfer them to the new reader.
Both Amazon and Kobo have many free books you can download from their stores. There's also Project Gutenberg where you can find thousands of free books. I haven't touched a paper book since 2010 and definitely do not miss them at all. Adjust font to a size that's comfortable for you, built in dictionaries, long battery life, never run out of something to read... what's not to love about ereaders.
