Obviously there is a lot that goes into a successful business plan, but a couple of things in your post concern me.
First, although I believe you may be able to use factory factory parts, charge less than the dealer, and be profitable, lower price should not be your focus. Even in today's economy, price is way down on the list of customer concerns with auto repair. Value is another story. You need to know what your cost of operation is in order to determine your pricing structure. If your less than the the dealer, that's fine, but I don't think that should be your focus.
Specializing is another subject altogether, you will start out that way, but if you are successful, your customers will undoubtedly own other brands that they will want you to work on as well. And especially during start-up or slow times you will be temped to take on other lines as well. Also not a problem if you are willing to tool up and make sure you have access to the technical information you need.
Good luck to you, I admire your desire to offer a better level of service to the vehicle owners in your area.