Responding to your last couple of questions because every else seems to have covered the ePub situation.
1) Menus: It's a simple text listing, that looks like this:
Code:
BOOK TITLE AUTHOR NAME
.................
BOOK TITLE CUT OFF IF TOO LONG... AUTHOR NAME
.......
You can sort books by author, title, or "recently added", and while it doesn't group by series, you can put your books into "Collections" to organize them. Collections are single-level only, though, no nesting yet.
Marking books as read is tricky. Under the title, you get a line of dots indicating how far you've gotten into a book (progress is marked by bolded dots compared to the total). Once all the dots are bolded, the book is considered "read", but there's no more visible way to do so.
You can search for a certain book, but be warned, the Kindle tends to search for all books containing your search term when you do it from the home page. Might work differently in Collections.
However, you do have the option when sorted by title or author to type the first letter and press the 5-way controller button, and it will automatically jump to the "C" section or whatever and you can browse from there. Same sort of shortcut works for numerically ordered pages, so you can skip to the 29th page in your listing of authors/titles/recent adds.
3) The buttons are reasonably convenient. I have the older K2 model, and while the K3 improves on it by having Prev/Next buttons on both sides (instead of Prev/Next on the left and Home/Next on the right), it does make the buttons narrower. You may or may not prefer this as being easier to grasp without clicking, or too shallow, depending on your perspective.
They don't seem to make much noise, but perhaps I've simply gotten used to it.
I think you may perhaps want a Sony for touch screen and ePub support, and possible expandability. No WiFi, though.
Hope this helps, and welcome to MobileRead!