I'm having a strange issue on Linux that all the buttons in Kerbal Engineer Redux seem to be empty:

What font should be used there and may be missing from my system?

I had the same issue. You can fix it by installing fonts through winetricks.
I didn't bother with checking which font was missing so I just installed allfonts
After setting the game WINEPRIFIX run:
winetricks -q allfonts
Or through winetricks gui in case you want to select which font to install:
winetricks>install an application>cancel>install a font>allfonts