Ok, so how big of a difference is the 10m vs 12m when it comes to wind range?
The two sizes overlap in wind range, but the problem is that if your local wind rarely exceeds 17mph as you said earlier then you will be mostly flying at the lower end of the 12m and below where the 10m will be able to pull you. If you are going to start with only one kite, a 12m makes way more sense. In fact, looking at wind charts for Lake Ray Hubbard, a 14m might be an even better choice but might be a little big if you travel to South Padre Island or elsewhere. Starting with a 14m and adding the 10m later would be good.