My experience is that ereader-store.de is very responsive and supportive and Booxstore here at the forum is always present, trying to help, answering questions and requests and putting up firmware updates (it is my understanding that they participate in the process of firmware development). So, I think it would be foolish to buy from anywhere else!
As for ordering a spare battery along with the unit, it is what I did. I have this experience with cell phones and a book reader like M92 should not be any different. One should buy a spare battery with the reader or shortly after, within six months. Batteries for old units tend to be difficult to find, if at all. And no, my experience is that lithium batteries do not degrade significantly after a couple of years of inactivity. It might not be charged, but after a couple of charges it should be OK.