I personally hate the front A2 sight. I dislike having anything appear in my optic, and it limits my rail choice.
As others have stated in this thread, they prefer them. It's really all about what works for you.
All your cost is going to be in your barrel, rail(depending on what route you go) and your trigger. Everything else is going to be ground noise in comparison.
Pay attention if the parts are made for mil spec or commercial. Those aren't grades of quality, they are dimensional standards. So milspec parts won't interchange with commercial. I find most people prefer milspec because there are more parts available.
I'll probably get flack for this, but I don't have backup irons on any of my guns. I just run a single optic.
If I were you i'd build a lower and then go shoot some different people's uppers to see what you prefer. Then build your upper based off that.