As I've said, I haven't taken any classes, but have spent many afternoons dredging through the mud with my AR and have put a thousand or more rounds through it in a relatively short period of time.
I'm no pro, but can make a few observations because I've suffered my own growing pains with the AR platform in terms of failures. From what I've seen, as long as you get a quality barrel, gas block and upper and lower receivers, the stage is set for a good dependable rifle. Where rifles usually fail is in the small parts. I replace extractors periodically and always have a couple on hand. I also replace extractor springs. Don't ignore the ejector either. Keep ejectors and springs on hand. Trigger parts are also crucial, or more specifically the disconnector and springs.
Although I don't keep my rifles spic and span clean all of the time, I do like to help to decrease the likelihood of failure by using quality small parts. I get everything having to do with the bolt from Bravo Company. I'm not saying they are the only game, but they are who I deal with because they sell quality parts and I've never had a premature problem with any of them.
With regard to the fire control groups, don't ignore the pins. Some guys buy the cheapest lower parts kit they can find and then are surprised when springs and pins break. If you want to go stock, again just stick with what's good. BCM, Spikes, DD, etc. I run Geiselle triggers and they've seen many thousands of rounds without issue. I'd also stay away from any sort of pins that require a tool to remove them. Having them can be the difference between being up and running again in a couple of minutes and having to head home.
It's also good to have a back up bolt. Swapping is faster than fixing. BTW, don't ignore the gas rings. Get good ones. I've been using that once piece ring that replaces all three and, although they don't work any better than stock, they do last a long time.
Given enough time, any part can break. All you can do is try to minimize the chances of it happening. Again, I can't express enough the importance of good springs. I've seen many guys, myself included, tearing their hair out because of a bad/weak spring.
Even if you go out and buy the best of the best and it runs like a champ out of the box, the time will come when you have to replace parts. All that I said above applies. You don't want to take something that works and turn it into something that doesn't.