Re
I am going to assume Eadric is planning to take this template. My honest opinion as a DM is that the template will not have an appreciable effect on his power due to the already potent items he wields.
+3 is about right IMO, maybe +2 if you are willing to play around with the ECL. His best spell is Holy Word and Ressurection, which the foes you face will barely be affected by unless he bypasses their SR.
The stat bonuses and immunities will be helpful, but your game doesn't seem all that combat oriented anyhow. They fight alot of devils with access to fire, which a half-celestial is still affected by.
I just don't think at your current level of play with the potencty of 20th level characters that the half-celestial template will be much more than window dressing with abilities that might save their lives for a time or two.
Not to mention, once you are a half-celestial, you can no longer be ressurected save by a wish or miracle. That is a major disadvantage at high levels of play.