I'm a Sora fan, but considering that you plan to use bar end shifters, I would go up to 105 in the other componentry just because you're saving the money on the STI, so go ahead and go higher end on the other stuff. plus I have a weak spot for the black 105 stuff.
On the other hand the Sora will serve you just fine, and with bar end shifters, you have the option of using them in friction mode rather than indexed, so you have the ability to have very reliable shifting without worrying about the fine adjustments too much.
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Tom
"It hurts so good..."