Hello from Alabama. I recently bought an M&P 40. The business about safeties is very confusing. You can buy new M&P's without any safeties, with the mag safety (gun is safe when mag is removed), with a thumb safety, and/or with an internal lock that prevents the gun from firing until a key is entered and turned.
I ended up with the mag safety (which I like since I have had it on all previous pistols) and the internal lock. I had to take the internal lock in order to get a gun with the mag safety and the 15 round magazine (strange situations arise).
You'll find M&P owners have a variety of opinions on safeties. To make sense of it all, download a catalog and look at the summary pages for each caliber. The table shows which model number comes with which safeties, etc. Find the model you want and chase it down. You may have to make a trade off.
I just ignore the internal lock. It stays unlocked.