For a SD handgun, the common answer is the biggest caliber and frame you can comfortably carry safely. If you buy a system that is over $350 and use good name brand ammo, a semi auto will do just fine. Yes, there is a one in million chance you will have to clear a malfunction, but is that worth worrying about? The S&W M&P line is the bees knees. Since you are limited to 10 rounds the .45 is an excellent choice. The Springfield Hellcat is the same. The engineering is flawless and tames recoil very very well. You can't beat them for the price. Plus the still come standard with manual safeties. If you are going to spend the money on a glock or Sig I would get an FN: they are incredible machines, and it is essential a re-packaged Browning HI Power at half the cost. For a wheel gun, stick to 357. Charter Arms, S&W, EAA, even Taurus has fixed the quality issues of late: something with a 3 or 4 inch barrel. With a snub nose you trade too much power for not much more conceal-ability. SD ammo will run you around a buck a round. My personal favorite is Winchester Defender or Hornady FTX. Several companies offer balanced ammo now: the practice ammo feels like the defense ammo. A big plus because you should shoot 200 rounds to break the gun in and to familiarize yourself with it.
In a home situation magazine capacity in a pistol should be so that you can fight your way to a rifle. Despite what Hollyweird says handguns are not death rays. Most are underpowered for their purpose. Get a shotgun. I've seen in many places where #4 birdshot is still lethal to hoomans but should not go through two layers of drywall on the other side of a room.
Biggest thing is training. Know your systems inside and out and in your sleep. Since I don't get to shoot as much as I would like, instead of oil, I coat mine with white lithium spray.
Remember the 4 Rules, and take care of yourself.
