As Barry mentioned you can use Calibre to convert your ePubs to a format the Kindles will recognize. I would suggest converting to AZW3 rather than mobi though. AZW3 is a newer format and supports features that mobi doesn't. If your books have DRM, you'll need to do add plug ins to Calibre to remove that before converting--but once the plugins are installed in Calibre the conversion process will be seamless.
I think the Kindle 7 would be a fine e-reader--but if you want to be able to read where there is limited lighting, you might want to spend a little extra and go with the Kindle Paper White or one of the lighted Kobos.
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