Re
I play a Chosen of Mystra in a FR campaign. They are +4 ECL. The ECL is listed at the bottom of the page that Mystra is on.
They gain the following: +10 enchancement bonus to Constitution, Silver Fire: 2nd Version detailed in FRCS, Immune to aging, disease and disintegration, Gain 1 spell slot of each spell level, Immune to one spell of each level as if permanent Spell Immunity spell were cast on individual.
They are very powerful, but CR+4 or +5 is about right.
Now, I don't do things game balance wise. I still advise the following:
I would only allow this kind of character to be played by a good roleplayer who knew the Realms. Part of the restriction of playing a Chosen is not listed in the rules.
The restriction is summed up as follows:
You are a servant of Mystra, chosen to protect, promote and enhance the Art in the Realms. You throw down evil wizards, and you don't use magic to dominate or do evil in the world, though you may not be good.
You should always be wary of using your power or throwing your weight around. You have been entrusted by a goddess with greater power than others, do not abuse that trust.
I would never let a power gamer play a Chosen of Mystra in one of my campaigns. I would never let a player play a Chosen of Mystra without a well-thought out background that gives me a reason why Mystra chose to gift them with such power.
When they are playing the character, I would expect them to follow Mystra's dogma and to play their character in an introspective manner when wielding their magical power.
These are the caveats I recommend when allowing a player to play a Chosen of Mystra.
I don't allow non-arcane casters to play Chosen, and I don't recommend that others do either.