You need the same amount of ambient light you would need for a paper book, the screen is not backlit unless you get a Fire.
Personally, I don't see a huge reason to repurchase most paper books I already own, as I already own them in a readable form. Books in an ongoing series...I am rebuying, gradually, e JD Robb series and the Lee Child Reacher series as all future books in that series will be purchased on Kindle. So, no, I'm not annoyed. It's my choice to buy or not.
The books (except some in PDF format, reflow on the page based on the font size selected, so it's not a problem.
Jump in, the water's fine!

And welcome to KindleBoards!
Betsy