My standard reply....
You're in an enviable situation, or at least, so I'm thinking from your post. If you are going to put on "A list" components and parts, I'm thinking you're willing to drop a bundle on your bike, and you're gonna ride it a lot and enjoy it.
Due to your back issues (I have more than my share of physical ailments as well), you may be the perfect candidate for a custom frameset. Getting a really nice high end CF (or steel, or Ti, etc) frameset made just for you, taking into account your measurements, flexibility, riding style, wants, and needs is a fantastic experience. I've gone that route twice now and am extremely happy with both bikes.
There are several manufacturers that make these, and stand behind them. Guru, Cyfac, Sarto, Serotta, Calfee are the first that come to mind. Feel free to PM me if you have any questions about the process, or how I think it ought to go based on my experience.
I'm sure many will disagree with me. And in the end, it's your money and you need to be happy with your decision.