I suppose I would start with asking what it is that you like better about your Roubaix than your Secteur?
Can your Secteur fit fatter tires? Different Seat?
Before you solve the question about what is better about the Roubaix than the Secteur, it is pointless to shop for a new bike, especially online (never hurts to browse your favorite bike stores).
Most of my riding has been on a Colnago Super, which I've had a few 100+ mile rides, and a few overnight rides. It has a steel frame, and I've connected a rack into the triangle hole of the campy-style dropouts, and with P-Clips at the top. The rack is a bit low, with a bit of an angle, but it has been very stable, and has treated me well. I just don't worry about the extra half pound of weight of the rack on days I don't need it.
I'm not sure I'd Jerry-Rig a rack to a Carbon Fiber bike, but I would have no problems with adding a rack to just about any other bike for the purpose of light touring. Long touring? Perhaps a touring specific bike, but the old Colnago has treated me well. I now have a cargo bike for heavy loads around town, but I wouldn't take it touring, at least not mine.