Ok, first of all, congrats on your glock. The 19 is THE perfect fighting pistol bar none. And the Gen III's happen to be my favorite. I own 2 glocks, a 23 and a 27. The 27 is a baby glock and the 23 is the exact same size as your 19. Both of mine are chambered in 40 S&W. I chose the 40 because I can get the ammo for next to nothing. If it weren't for that, I'd have gotten 9mm's. The 9mm is an outstanding round. With today's modern technology, some 9mm hollow points can match and even exceed the performance of 40 S&W and 45 acp. 40's and 45's are great guns, don't get me wrong, but anyone that can shoot a 40 or 45 well can shoot a 9mm better. That is not an opinion, it is fact. The 9mm has more velocity, equal (and in some cases better) stopping power, and more capacity than any 40 or 45 does. Also fact. Not to mention the readily available ammunition and lower ammunition cost than either of the others. (aside from the present panic condition of this country). In other words, under NORMAL circumstances, 9mm is cheaper and more readily available than the others. But enough about your choice of caliber lets talk about the gun itself.
The gun itself, if you didn't already know, is excellent. To put this into perspective, I'll tell you this: I own a Kimber, an HK, a Browning Hi Power, and a few other pistols costing at or over $1000. They all stay in my safe because I carry my Glocks. The reason is simple: reliability. When the **** gets rough, there are only 2 guns under the $1000 mark that will keep running. They are glocks and smith & wesson M&P's. There are plenty of other good guns out there, don't get me wrong, but a good gun to me involves several categories: accuracy, reliability, durability, factory support, and aftermarket support. The other good guns are lacking in at least one of these categories. I love sigs. But if you can't get parts, what good is it? I love my HK, but the factory has no support for civilian sales. I love rugers too but good luck finding aftermarket sights, holsters, etc. You could buy a $3000 1911 that passes all those categories, but if you actually need it to save your life, are you going to be able to just go to the store and buy another one when yours is taken as evidence? Are you gonna be scared to get it dirty? scratched? used? The bottom line is, Glock or M&P is the only way to go for a carry gun. That being said... Let's talk about upgrades. You did say you're going to carry it.
If you're going to carry it, you want night sights. That means XS Big Dot Tritium sights. Try them, get used to them, you will love them. And while we are on the subject of upgrades, I'll say this: Do not upgrade anything else. No beavertails, no extended anything, and don't change the trigger. Glocks are as reliable as they are for a reason. When you start changing things, you start having problems. Your mag will accidentally drop in the mall because your extended mag release pressed against your holster... The slide will lock back when you don't want it to because with your high grip, you bumped it with your hand while the gun was recoiling.... Don't change stuff that already works. That being said, wolff +10% magazine springs are good. Do not swap out the stock ones for the sake of upgrading, but in 10 yrs or so when the stock ones wear out, buy the wolff's to replace them. And speaking of mags, you do not need an extended one unless you want it for the cool factor. For carry, Keep the 19 mag in the gun and a 19 mag as backup. If you want a couple extra rounds, pick up a Glock 17 mag to carry as your spare. That'll hold an extra couple rounds right there.
Holsters: You said you have to open carry so I'll stick to those. Kydex is your friend. It will not collapse like leather does and prevent re-holstering your gun. Will you be concerned about reholstering if you're in a shooting? Probably not. But you're going to spend a lot of time on the range practicing right? Nod your head yes. So kydex is a go. Will it cause more wear to the guns finish than leather? Probably. But you bought a glock, don't worry about it. The factory finish is hard as nails. You'll wear the finish off the top of your sights and have your slide stop lever worn down to bare metal long before your slide starts to show signs of holster wear from kydex. Trust me. Since you're open carrying, I would recommend a minimum Level II retention holster, a level III if you can afford it. Brand? Not sure. I've heard the safarilands are good. I've owned a level II blackhawk and it was great. Whichever one you choose, just practice the crap out of your draw. Don't stop until deactivating the holster safeties and drawing are one smooth unconscious motion.
Safety. Here's an important one. There is only one safety you should rely on: your finger. Keep it of the trigger until you're ready to shoot and you won't have any problems. Those guys telling you not to carry one in the pipe are idiots. THAT is dangerous. You're carrying this gun to potentially save your life. What are you gonna do? "excuse me mr. bad guy, can you stop trying to stab me for a moment while I chamber a round?" Not gonna happen. Carry condition one. Loaded mag, loaded chamber. Keep your finger off the trigger and you won't have a problem. Guns do not shoot themselves. I don't care what the liberals say.
The guy talking about the trigger safety being activated and having to cock it back to be able to shoot doesn't know what he's talking about. The trigger safety is passive. It is deactivated when your finger touches the trigger. Cocking it back to be able to shoot doesn't make any sense. You know as well as I do that the glock doesn't have an external hammer to cock. It has an internal striker. If he's referring to chambering a round as "cocking" he's still an idiot. Carrying with an empty chamber is called condition two. We've already established that that is dangerous.
There seems to be a lot of this type of bad advice and info being given here. In fact, I've seen the absolute worst bad, dangerous, and just plain wrong info given here. And that includes the moderators. Basically this site appeals to new shooters. And that's a good thing as long as you understand it. The new shooters wanna talk about guns, they type in "firearms" into their smart phones and this nifty little app comes up. Woohoo! A place to meet and talk about guns. That's awesome. The problem is when these new shooters start trying to give each other advice. Not awesome. They don't have the experience necessary to be giving any advice. I'm not talking about EVERYBODY here, but the armchair commandos and mall ninjas out there know who you are.
But there are other places to get better info. www.gunbroker.com
has a forums section visited by gun guys from every aspect of firearms use. From collecting, target shooting, trap, sporting clays, 3 gun, defensive, precision, etc. They're all there. And they have much better info to share.
There is also www.getoffthex.com
That site is ran by James Yeager who owns and operates Tactical Response in Camden, TN. That site is focused around gun FIGHTING. And those guys know their ****. If you go there, read the instructions on the home page first. By the way, you should do a Youtube search for "james yeager" and check out his page. He can come across as an *******, but he knows what he's talking about and he has a TON of useful, helpful, and informative videos about carry guns, carry methods, shooting techniques, you name it. You can learn a LOT from him.
I've been carrying for a long time. I've been a "gun guy" for even longer. I've spent countless hours researching answers to many of the questions you've asked and several more. So any of you guys out there who are offended by something I said can go fly a kite. Seriously, I could care less.