Andi,

Be very carefull using the data supplied by the manufacturers of the prepreg material. The conditions, techniques etc of how the laminate is laid and applied can change the end product physiscals by as much as 40%. The advantage as you said is that you can easily add additional material latter, but it will be difficult to know when, as the typical failure with a carbon laminate is catastophic.

The stiffness of the carbon fibers will probably easily give you enough stiffness, but the point of failure will be from compression along the top of the cross bar. Do what you can to maximixe the thickness of the laminate without adding too much weight, as thicker laminates provide better compression resistance.

Good Luck
Matt