AppleScript: Select a specific drop down menu item?
As far as I can guess at your "structures", code to select a specific font might look like this:

-- insert after: click menu item "Set Font..." of menu "Edit" of menu bar 1

click pop up button of group of window "Set Font"
click menu item "[someFontName]" of menu of pop up button of group of ¬
   window "Set Font"


Depending on dialog window's "surroundings" s.th. like "group 1 of..." may be necessary.