Brake Master Cylinder problems of the 2009 GMC Acadia

We've had electrical issues beginning about 2 years after buying the Acadia. Currently the check engine light stays on. We had it checked at the dealership and were told that it was safe to drive. The problem was probably part of the electrical issues. Well, it wasn't. Now the steering is difficult (and locks up at low speed turns) and the stabilitrek message comes on. We took it back to the dealership today and have been told that we need to pay for repairs to service the brake system, stabilitrek message, replace the brake power booster and master cylinder, and repair the tire pressure system light and sensor. They think this may fix the steering issue, but if not then they'll look more into that! I know there are at least 195 service bulletins for this year model Acadia. We bought it new. It's seven years old with low mileage. They said it isn't safe to drive except in town at low speeds. I feel like we're being given the run-around by gm via our dealership. It's never been in an accident (other than knocking off the side mirror upon exiting the garage), and it's not driven in a rough-shod manner. Ugh! I've owned several vehicles, new and used, and never had trouble like this. My daughter is the primary driver of the Acadia, so safety is of the utmost importance.