Neither the Fairphone shop nor two (German) resellers that I typically check as alternatives carry the FP2 battery anymore. Buying a fresh battery for the FP2 could turn out a showstopper.
As to the question which other model to choose nowadays, I guess this topic could help: FP6 vs FP5, thoughts?
Just to support your decision to go for a new phone.
I’ve been using an FP2 for almost 6 years.
In spite the battery was still in good shape, I decided to get a new phone.
Bottom line: Without spending time and having the technical skills to set up and tweak certain niche mobile OSes (and still making compromises in everyday use), you can’t run the old hardware with up-to-date (read: reasonably secure) software any more.