Here are the Unicode codepages for
Greek &
Greek Extended with decimal, hex, and HTML entity equivalents if applicable.
The Summer Institute of Linguistics produces
freely-reusable freebie fonts specifically for representing Roman, Greek, and Cyrillic-alphabet mixed multilingual documents. I've always thought Gentium was the nicest looking one.
As for Mobi version conversion, Greek characters (including polytonic) have been supported in the internal font since the K2, and as long as you're outputting as Unicode everything should display fine.