Barbarians traditionally only wear medium armor at first, later upgrading most of the time to mithril heavy armor... Which Warforged can't really do, since they have their armor built in and have to chose at level one and can't change later. Which makes being a Barbarian really really hard, because if you go for the light option later on in the game you will have a lower then normal AC whereas if you take the heavy choice you can't ever use fast movement and you will have non-proficiency penalties unless you get the feat. So. Sub par Barbarian or Normal fighter... And later on of course you will be hurting because other fighters effectively have one more feat then you, once you get to the point where 1,500 GP is not a huge number.
Damned if you do damned if you don't I guess... I hope that helps you chose?
He should take the cheesy way out and become a Warforged Juggernaut.
As for the Silver Flame, I'd say it is probably a farse, much as Tira may be getting a Divine Rank of 0 out of it.
It is unlikely that Eberron has greater deities at all; it is only a bit more highly powered than Dragonlance, where a level 19 wizard was capable of slaying the whole local pantheon.
Trivia: Ironically enough, Corellon is far more powerful than Ao outside Albeir Toril's crystal sphere. That goes to show how a deity's power is relative not only to its Divine Rank, but the planar battlefield and the deity's inherent power. Likewise, considering his lost eye and hand became major artifacts before he had even ascended, Vecna, in spite of no longer being a greater, is probably amongst the deities with the most class levels.
The "most powerful" deity I am aware of is Jergal at his prime. Even divided his portfolios were still capable of spawning three greater deities.