You should go to a reputable fitter to get pre-fit to find out what bike geometry will work best for you. The fitter should have some kind of "fitting bike" they can adjust to do things like change the top tube length, bar height, stem length, etc. That way they can adjust the bike close to the bike you want to purchase. Here's an example of a couple of fitting bikes:
http://www.next-fit.com/fitbike.html
http://www.cartecaybikes.com/fitmaster-machine/
Since you don't already own a road bike this will help determine the correct size for you. Beyond just measuring your height of your crotch and various other measurements. For lower back issue I think the most important measurement is the head tube height of the frame. The taller the head tube, the less strain you are putting on your lower back/core. With a taller head tube, it gives you more options to lower and raise your stem/handlebar height. Without compromising the safety of your fork. Most forks have a maximum height which they can be cut above the head tube & head set. I don't know what your height is but I would look at frames that have a taller head tube. You will still be just as fast, but you will be able to get the bars at the right height. And if you start out a bit above where you need to be, you can bring the bars down over time as your flexibility and strength improve. A frame like this would be a Cervelo RS or Parlee Z5 (tall frame size). Maybe these are outside of your budget, but the fitter should be able to figure out the best frame size and head tube height. Then you can shop around which means these specifications. When you see someone with their stem flipped up which is not uncommon or bad, but this generally means the frame maybe a bit small for them, their flexibility/core strength needs to be better or their fork is cut too low.
As someone who has lower back pain, having a bit taller head tube helps so I can not only ride on the hoods comfortably but ride in the drops comfortably too. I like to race, and do very hilly rides with lots of descending. Which puts me in the drops a lot and if I am too low I can get lower back pain. A CAAD9 is a very nice frame, just make sure you get a proper fit prior to buying the bike and then take it back to the same fitter for the final adjustment. And make sure the bike shop doesn't cut your steerer tube too short on your bike.