Just a few notes: Yes, there are plenty of used ones. Just check Finnish resell websites (like huuto.net) - last time I checked (few months ago), you could get one for less than 100EUR. I don't live anywhere near Finland, but sending it to other countries was no problem.
Just a few notes: Yes, there is. I am now happily using two batteries in my Jolla and I'm thinking of getting a 3rd one. The new one was for under 10EUR and it works better than the current official one (as it is brand new, unlike the other one).
Just a few notes: Good luck with that choice. It wouldn't be me first - or even a second or third - choice because of Ubuntu Touch (many, many reasons why not) and the lack of 4G. If not for another Jolla, I would go for the Nexus 4/5 and run either SailfishOS, LuneOS, or Plasma Phone on it.