If you find the recent Github issue where hyphenation gets set based on the language of the ebook, the "merge" post will have links to where the scripting of the files is spelled out.
Koreader uses several font types but .ttf fonts into the "freefont" folder, is the easiest way to do it. (My favorite is Stone Serif Regular.ttf)
Luck;
Ken
P.S.; If I were going to buy a font it would have to be something like
this $25 font.