Look up a good article on coding fonts and pick your camp! At the moment I have DejaVu installed but I'm not a purist. As long as it's properly designed for this I'm happy. Ligatures are particularly nifty in some languages but no big deal. I recall one author picking a font so that the italics would be cursive rather than monospace, so that his comments would look like handwritten notes in the margin, but I never got a chance to try it myself. Looked great though!