Glad to hear you are all getting along with the Cabrinhas well. I went for this system also. However, I had to go for a further kite!!
I now have
There is an overlap with the 17m CO2 and 14m Nitro if you look at the tables, but I phoned around and a very experienced instructor told me that those table are so arbitrary that you should still think in terms of size, one for one, and worry less about the models than a lot of the literature would advise.
I think that the 17m CO2 is as big as I would go with the CO2. They do jellyfish if you get beyond this. The contra is designed for lighter winds.
Right now, if I were to buy again, I might reconsider the 14m / 12m choice, but would stick to the big CO2 and smaller Nitro. If you live in an area with decent wind, a 12 would be good. It is a more complete quiver, different feel to the kites, good for getting to know what you like.
It pays to shop around. There are several places that are doing the 2000USD for 2 kites, and board. Haggle. I saved 150 bucks. (DVDs, sand bags etc)
I'm 215 lbs, 140 board, and needed 14mph for the CO2 just to stay x-wind. This is why I went for the contra also. Cabrinha told me that the 19.5 can produce as much lift as the 23.5 if you can fly it well, but if you are heavy and fairly new to the sport, as I am - go for the 23.5.
The contra should go with the larger bar - 58cm. The CO2 flies ok with the 51cm, but I would prefer a larger bar.