I have the DXG so I cannot speak for the K3, although the two are pretty similar.
The buttons do not make much noise and the menu us very easy to use. You can build your own collections. Essentially you are tagging books so they can be organized in a manner of your choosing. I have created collections for different genres that read To be Read Sci Fi and Read Sci Fi. I move the book after I have read it.
If you are a heavy PDF reader then storage space might be an issue. I have 400+ books on my DXG and am no where close to maxing it out. It can hold 3,500 books. The K3 has the same capacity.
Converting EPub to Mobi is suppose to be easy to do. Do a quick google search and you will figure it out quickly enough. Several folks who have moved from Sonys or Nooks to Kindle have said it was easy and not all that time consuming.
When you buy the device and the book the terms of service say you cannot strip the drm from books. Plenty of folks hear will argue that the rule regarding books that you buy has always been that you can do with them as you please as long as it is for your use only. As long as it is for your own use you can photocopy or scan a paper book you own, similarly you can strip the drm off of an e-book that you own.
That is an ethical/moral/legal debate you will have to wage with yourself.
Personally. Amazon has all the books I want. If they didn't and I found a book in another location I have a feeling I would be fine with stripping the DRM and converting it. I have not had to do this yet.