So, to answer your questions directly:
1. I would be looking for a good quality, double-stack 9mm, hammer-fired with some type of external safety or de-cocker. Personally, I like the CZ 75B, CZ Shadow. If you are trying to stay around $500, I wouldn't hesitate buying an older S&W 5906 or even a 659. The guns I've mentioned are extremely reliable, durable and easy to operate.
2. I prefer 9mm over 45 because ammo is substantially cheaper to buy in bulk and practice with. For home defense, 9mm defensive rounds are perfectly fine.
3. I do not like Glocks or Glock-a-likes for first-time gun owners - most will refuse to keep a round chambered, and even if they do, and the gun is unholstered, its just too easy to grab the trigger and BANG!
4. Optics - Unless you are well trained with a red dot optic on a handgun, they are much harder to use than you might think. Pick the gun up and if you can't find the dot, then what? Keep searching for the dot??? For me, I can't stand red-dot optics on handguns, but then again, I haven't spent hours upon hours training with one.
5. If you get a 1911 for home defense, remember you must keep a round in the pipe, hammer cocked and safety on. As a new gun owners, are you going to be "ok" with that?