I have a Kindle Paperwhite. Bought it some 3/4 years ago… (woah I feel old)
You can add any file that can be read by the Kindle to the Kindle. Even if you didn’t acquire it through amazon.
However, if you buy an Amazon ebook, you can’t read it in a Kobo (another kind of e reader), for example. And vice versa.
Tbh, the Kindle has been one of the best purchases I’ve ever made and it serves me really well to this day. I highly recommend it.
A tip is not to go with the simplest kindle (I forget the model name) because it doesn’t have built-in light. You want that otherwise you’ll bee too restricted in your night readings and will have to rely on lamps etc.
No need to get anything fancier than the Paperwhite, though. I don’t even know why voyage is a thing tbh unless you want to look fancy and get some buttons on the side.