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The M&P is a great choice, but acquiring a weapon is only the first step.
I would recommend you take a tactical training course. They have a number of good courses that teach tactics in close quarters and vehicles which would give you a lot of knowledge and a better understanding of some of the dynamics you might encounter. It will also help you develop an understanding of your personal assets and liabilities and may well change your opinion on what kind of a weapon you want. These can be kind of expensive, especially if you have to travel, but the information you will get from a good course will be invaluable in the event you need it.
Good luck!