I gotta say that I have alot more sympathy for the gamers that want to look 'kewl' than those that want 'kewl' abilities.
The decision to have non-evil undead, and especially non-evil death knights (which were originally all CE I believe) is pretty campaign specific, but hey, if your DM thinks having Death Knights as the servants of a LN goddess of death is cool, I can see where he (or she) is coming from. I'd probably also work it up as more of a prestige class than a template, so it sounds like you are in good hands.
I'm courious as to just who the goddess sends you after in your capacity of 'angel of death' and why? Do you primarily slay other undead, those that try to thwart the Goddess in her divine duty of collecting the souls of the dead for passage into the next life? I would think that as a servant of a LN goddess of death, there would be severe limitations on who you could kill, because hey, 'Fate says it is not your time yet.'