1. I'm a member of both. DFW AC is free (just go the site and register). It's a loose Internet based all makes enthusiast group. The membership is interesting and diverse from wealthy guys with the latest exotics to poor students. There is lots of chatter on the website. If you have a question, someone will have the answer or know where you can find it. DFW AC members are into DE's, drag racing, motorcycles, guns, getting together, and lots of other stuff. BMWCCA (not free) is more formal. The site and forum are not very active, but as I said before, the chapter is very active with monthly autocrosses (spring - fall), a couple of DE's a year, and tours. Non-members are generally welcome to participate in BMWCCA events.
2. There are rules about when you should get a state driver's license and register your car when you move to the state. You'll need to look at the TX government sites to find out what those rules are. My guess is that you are fine (for practical purposes) until your Kansas stuff runs out. My experience is that the Texas DPS (driver's license) is painful. The county tax office handles vehicle registration. That's not too bad (at least up in Denton). I'm pretty sure you will have to have a Texas inspection before you can register your car in Texas.
Yeah, no doubt we'll cross paths.