99.9% of AR stocks are adjustable. If the barrel is marked 5.56 it will shoot both calibers. Only barrels marked .223 are .223 only. Still no need for 18" barrel. Upgraded trigger just gives a smooth and lighter pull. If your not an avid shooter you probably won't notice the difference. And on a semi auto you have to be very careful with to light of a trigger. You can send double taps down range when you don't want to. Can be very dangerous
Here is a video. Don't get fixed on the prices because AR builds have thousands of varieties and cost can stay mild to wild