I have both an SD-E on an AR and the Super SCAR on a Mk17S and i highly encourage you to look into the flat bow options over the SSA-E. There is a noticeable difference using the mechanical advantage of the lever. Other than a proper trigger, you’ll want to do a lot of research to try to make the platform rattle, wobble, jiggle, or flex as little as possible to get the most out of your build. You may also want to get a fancier charging handle like a raptor or similar ambi model, and a NiB BCG (http://emptormaven.com/2014/02/the-problem-with-nickel-boron/)
Contrary to popular belief, mil-spec tolerances weren’t/aren’t the greatest for what is the intended usage for civilian rifles. Plus we probably clean ours more often than the average GI (unless they’re in the sandbox or off in the mountains of Afg)
To echo an earlier comment, most of our rifles will NEVER see the number of rounds or circumstances for which higher-end manufacturers or accessories are designed. Although it can’t hurt anything but your wallet to get them.
Geissele also as a S3G (three-gun) trigger that utilizes the flat trigger.
I’ve been building my ARs since 1995, so I don’t slap in parts willy nilly. The only “fancy” charging handle I use is the original Badger Ordinance Tac-Latch, which gives me greater leverage when I need it.
“mil-spec”: the minimum becomes the maximum, circa 1964.