Here is my take on this issue after using this platform in both civie and military for 8 years now.
Safety: From my experience the only part you actually ''need'' is the ambi safety. Although, you can survive with a right handed only safety too, it will just take some getting used to and won't be as fast.
Charging handle latch: The second part you'll want is a new charging handle latch so you can grasp it more easily like a right handed shooter would. Again, you can live without it too as there are methods (finger placement on the latch) left handed shooters can use to negate this disadvantage.
Mag release: I find the add-on mag releases clumsy because they don't have the metal housing built around them to help prevent you from accidentally pressing on them and subsequently dropping a mag. I've seen a few guys lose their mags on their C7A2s for that reason. Furthermore, I actuate the mag release with my right thumb as I drop a used mag so I really see no need for an ambi mag release. However, if I would consider one, I'd go with Knight's Armament because they have a truly ambi lower receiver that negates my problems with add-on mag releases.
Ejection port: AR15's launch their spent brass ahead and to the right, so you really shouldn't have any issues. If some brass makes it to the rear, it will be deflected by the brass deflector (not a bad idea btw). The only issue I can think of with this is powder. Sometimes I get a black smudge on my face because powder escaping the ejection port has made it to my face. As long as you wear eye protection this cannot hurt.
Sling mounts:If you're serious about slings you're going to want a 2 point sling (or a single point sling if you have a carbine but I still prefer 2 point slings for both) who's front end is secured with a mounting fixed onto the right side of your rifle. I have one an my RIS on the 3 o'clock position. In regards to the rear end of the sling I strongly suggest you don't mount it on a sling mount built into the receiver end plate as it will get caught on your charging handle, your forward assist, your ejection port, it is just obnoxious (saying this because I've tried it of course). The best way I have found to secure the rear of a 2 point sling would be on the left side of my LMT Sopmod stock with a QD swivel. It never gets in the way nor tangles. If your stock doesn't have a QD swivel mount, Knights Armament sells an adapter. Most stocks have sling loops built into them so there are several options available.
What do I have on my AR's? I have three of the above installed. Ambi safety, ambi charging handle latch and a sling mount on the right sight of my RIS with another QD swivel on my butt stock. I can detect no handicap from my part in comparison to right handed shooters with those three parts.
Left handed uppers: Good idea. However, you're gonna pay more for them, you're gonna be stuck with that original manufacturer for parts when they eventually crap out on you, and resale value will be horrible because who wants a left handed AR?
My 0.02$
