e.lectronick:
Welcome aboard!
To be honest, IMHO, both the magazine and thumb safeties aren't necessary in the M&P design.
To add a magazine safety, though, you need a couple of little springs, and a little lever. Probably cost more to ship 'em than to buy the actual parts. Pretty much nothing to it if you are used to mucking about with little bits of metal and pesky springs. Either sear block assembly can accept the magazine safety if it didn't come with one.
The thumb safety is another story.... You have to start with a sear block that is designed to accept one. There are a couple of little parts that have to be there, or the thing won't work. This precludes the Hilary Lock. (If you even
mention that to your wife, we'll toss you off the board
.) The "standard" sear block is designed to accept the Hilary Lock....
You should be able to buy a thumb-safety ready sear block from Speed Shooters, but I'm not sure. (Just FWIW, you can easily remove a thumb safety from a gun that came with one. Going the other way can be a real mess.)
If you add the mag safety, btw, your gun will still likely have the "this gun can be fired without a magazine" warning on the side. That tends to upset some of us....
(A "Hilary Lock" is a key-operated lock built into the gun to keep it from working when you need it. Just about any other "safe" sort of thing is a better idea, although it looks like the design used in the M&P would be reliable v.s. the ones used in at least some models of their revolvers. A "sear block" is a hunk of steel towards the rear of the gun that holds the sear, magazine safety lever, sear disabling lever - used to strip the gun for cleaning, thumb safety cam & spring - if you have that version, and the Hilary Lock.)
What I'd do.... Buy the thumb safety version and add the magazine safety. Some magic marker over the "message" and you're done. Five minute job if you can find the parts. The springs just hold things in place, and about anything will work, but the lever itself is a "part" you need to buy.
If you can't get a thumb safety model, talk to Speed Shooters and see if they have the block and the few extra parts involved. You will have to cut notches in the plastic grip assembly to fit the thumb safety lever, but that's pretty trivial. Adding a mag safety to that block wouldn't be a problem.
About the only real concern here, and I'm always preaching this, is to never remove a manufacturer-supplied safety device from a carry gun. IMHO, "civilian" Concealed Carry probably won't be an issue, but LEO's worry about this should they end up in Civil Court.
Regards,