My own suggestion would be the
Charis SIL font. It comes in the four typical styles (regular, bold, italic, bold italic) and has all the glyphs needed for transliteration. It is even attractive (enough -- in a chunky sort of way)! (IMO)
There is also
Gentium, but (a) its right- and left-half-ring for "alef" and "ayin" are a bit nasty (I think there is a plan to replace them in upcoming releases), and (b) it only has regular and italics (no bolds). (The
Gentium Basic family makes good on the bolds, but surprisingly lacks the right- and left-half-ring!)
The
DejaVu font family will, I think, also give you all required glyphs, and with many more style options.
Hope that helps! (A little late, I know, but I've just joined the forum!)
David.