J, there are some vendors that will put a lower parts kit together for you using the trigger kit of your choice. I believe that two such vendors are PKfirearms.com and Ranier Arms. If you just want a good solid run of the mill LPK, the Stags were the best bang for the buck the last time I was searching for a standard kit. The trigger feel ain't bad and all of the springs and pins fit right and did what they were supposed to.
If you decide that you want to go with an upgraded trigger, I can unreservedly recommend the Geissele SSA-E trigger for a field rifle. It has a VERY clean break. Not like a match trigger, but it's really nice without being overly light. I ran one on my last precision rig that I built and it worked out nicely. I also run one on my goto AR.
As for the trigger guard, I'd go with the plastic (polymer) one. The aluminum is nice, but it gets scratched up and dinged and what not. If you need the ambi safety, then go with it; but I prefer not to add too much to the controls of an AR. You never know when you'll need to do field repairs. The way an AR is designed, you can break it down to its major component parts using nothing more than the tip of a bullet. I don't like to add anything that changes this, such as anti roll pin sets and such.
I keep an little baggie full of replacement pins, springs and gas rings in my MOE grip and I don't have to worry about having the right tool to change any of them.
