I have not bought a Glock so I may not be able to answer all your questions about the 19 but in regard to the 15 round magazine, that is a state thing. California for example, limits magazines to 10 rounds. If they are selling 15 round mags in a legit gun store in your area then I would assume they are legal in your state.
I would wait until you've taken the course, and perhaps have rented some guns at your range before making a purchase. Basically all the guns you mentioned are quality firearms so it comes down to personal preference for certain features, how the gun feels to you and how well you shoot it. You already have a fairly clear idea of what you are using the gun for. As for the Glock 19 I would say it serves the purposes of HD, range tool, and occasional carry weapon.
Good luck and welcome to the forum!