Since you didn't mention CCW, I assume this will be a home defense/range pistol ? Before you get into buying accesories, buy a good supply of ammo and practice as much as possible. If you don't have alot a experience with fireams, get some good quality instruction. I would start off simple, then work you way into other accessories. For home defense, I would recommend night sights and a weapon flashlight. Lasers have their uses, but you cannot identify a target in the dark with one. I prefer Meprolight nightsights. They are less expensive than Trijicon and burn bright for years. For the price, you can't beat Streamlights TLR-3. You can get them all day for under $70. If you want a decent belt holster try Fobus. They are simple and tough(made in Israel). They also have mag carriers. Blackhawk also makes very good single/double mag carriers. If you want leather, try Galco or DeSantis. They're stuff is well made and reasonably priced.
As a fellow Kalifornian and a law enforcement officer, I advise you not to alter the firearm in any way. Adding a set of night sights is fine, but intenal alterations are trouble if you intend to use it for defense. Prosecuting attorneys have a field day with altered firearms. Trigger work is fine if it's to be used strictly for taget shooting, but you may need it one day to defend yourself.
Owning a gun is not cheap. Becoming proficient with one is even less so.
Nights sights aid in lining up your sights on a target at low light levels. They are fairly useless in total darkness(you must be able to ID the target). Some prefer a weapon mounted light, while others prefer a hand held light. I've trained and am comfortable with either. They both have their pros & cons. It comes down to how the firearm is used. Night sights are not worth the cost on a range only firearm, but as I said earlier, you might need it for defense, so why not have some cheap insurance ?
I have no issue with spending $50-$100 etc to upgrade a Glock trigger. I have certain preferences so that I'm going to change a number of things on pretty much anything I buy...that's life.
Go buy your G19, start shooting the heck out of it, if there's something you don't like you can change it.
So when do you know your experienced enough? I dont get the last comment.
Experiment to find the best holster location for yourself, and always carry your pistol in that same place.
If you switch it around, it won't be where you're reaching for it, when you need it in a save-your-life panic.
You also need to know how to strip, clean, and reassemble your gun, quickly and efficiently.
It helps, also, to know the applicable laws concerning guns, both state and federal.
A laser will not help you shoot. If you do not have control over your pistol, the laser will only "dance" all over your target. This is very confusing, visually, to the shooter.
A laser can help you to practice your trigger control. If you can keep the laser's spot in the same place, all the way through your trigger press, your shooting will improve.
A laser may seem worthwhile, but if your fundamental shooting skills are not fully developed (see "experienced," above), it is merely a confusing gimmick.
A wiser man than I once said: "No gimmick, gadget, or miracle-of-the-week will ever absolve you from the need to learn and to practice good fundamental shooting skills."
However, with enough time and training/practice I know many people that carry multiple ways. I know a lot of officers that have to carry strong side in a duty rig but carry other ways while off duty.
I've been complimented in the pat about how well I access my gun etc. and I attribute a lot of that to the gun being worn the same place regardless of which guns I'm carrying, I'm currently carrying in a different method than what has been the norm, but I make a point to do several draw-stroke presentations when holstering the gun and a couple "air gun draws" throughout the day as well as visualizing the drawing process and mentally reviewing my gun's position etc...
But I digress, the OP of this thread has about as much to worry about holsters as I have to worry about zombie's from Mars attacking on the 8th day of the week. HE LIVES IN CALIFORNIA IN LA COUNTY, HE WILL NOT BE GETTING A CARRY PERMIT, and I doubt very much that he will be putting in for any non-resident permits any time soon....
It's all good to think about, but lets not get overly fixated on holster issues until he at least buys a damn gun....I'm beginning to think the zombie martian attack is more likely to occur at this rate.
Thanks guys for the comments, you guys rock!