Cheap one will give you internet connection with WI Fi or router, the expensive one will give you IT connection, ANYWHERE.
No you cant send emails from it......only to Amazon via their special email address.
New books may be obtained from Amazon site, which have to be paid for.
You can get books FREE from websites but you need an intermediary on your computer e.g www.calibre.com to facilitate this move.
From Amazon, you do not need to plug in your Kindle, anywhere, but from other sites, you need to plug your Kindle into your PC.
Sorry this is non technical.;-)