I believe I have found what I am looking for!
I am looking to make a telescope of this size. I want to take photos with it on my GM-100 EQ Mount, and want to minimize coma, hence the f/6. I want to keep it easily useable on the mount for someone of my height (6'4"), hence the 12.5" I'd be interested in other combinations that come close, though nothing smaller than 12" aperture. I don't have a pile of money, but enough that I think I can make a deal. Of course, I'd be interested in a complete (or incomplete) OTA, if someone has one they can let go of for the amount of money I have available. I'm also willing to drive quite a distance to get it.