The non-K CPU's have locked multipliers, so that's why they can't be OC'd very far. As to whether the iGPU increases OC ability, no, but it doesn't decrease it any either. You can OC slightly with non-K i5's and i7's through the "Limited Unlocked" ability with Turbo, but only by 4 speed bins, or 400Mhz.
The 2500K is the obvious best choice because of the ability to OC when/if you need to.
You're correct that P67 doesn't allow the use of the iGPU, but Z68/Z77 are usually thought of as better boards overall, even though they don't increase OC ability (or decrease it).
It's a good idea to be able to use the iGPU if you need to for trouble shooting purposes in case of a discreet GPU failure, and so you won't have to go without a computer while you're dealing with a RMA (should the need ever arise).
My advice is to go with a 2500K/3570K and a Z77 board.