Just some answers to your question instead of a lecture...I'm sure you evaluated the price and the vehicle
You'll still need a brake controller. If I remember right (I had a similar suburban)it was just a simple plug in to a module by the brake pedal - had to order the right pigtail for ~$10 or so. I'm sure you'll love the Yukon XL - they make great tow vehicles for those of us that can't devote a dedicated dually 1 ton for a few trips a year!!
Extras that you can ask for: A few free oil changes or car washes are always nice to have around and the dealer should be able to throw those in pretty easy.
Or you could purchase a Class 8 Semi to tow with
