My concerns about AR trigger work are safety related...Unless you maintain the EXACT angles for both the hammer and trigger you can develop conditions that can easily result in a negligent discharge...be it a slam fire or just a hammer letting fly when you least expect it. Can you learn a lot polishing on one...yes.. ..but you will spend much more than you save if you purchase the correct stones and materials to make the jigs you'll need to develop the proper relationship of angles between the hammer and trigger.
If you have a home shop...have at it....but be aware that you are treading on thin ice when it comes to safety as well as longevity......most of the less expensive parts are made by MIM...metal injection molding, and when you cut through the outer hard shell it WILL wear at an accelerated rate...once again resulting in a potentially unsafe condition ranging from slam fires to potentially an unintended full auto runaway. I've been building AR's since the late 70's and am an experienced machinist with hundreds if not thousands of builds under my belt including ones where I designed and fabricated the entire fire control group.....and I don't mess with AR triggers.....just not worth the risk as you never KNOW what you are really dealing with. With something I design, I have the experience and metallurgical knowledge to source the proper materials and successfully heat treat them, with a cheap AR fire control you are dealing with unknown material of unknown metallurgical properties.