Not mention in the OP was what the intent for the weapon is.
The introduction of the piston operated, both long stroke and short stroke systems have definitely change the rules for the AR platform. The gas impingement's dirty operating method has been eliminated and has greatly improved the reliability of the AR. However the piston systems have created a wide gap between the cost of the 2 systems.
If the AR is used for plinking and target range operation and will be cleaned after every use, then A low cost impingement system would be okay.
If you're going into the field and exposing the AR to the elements and don't plan on putting a lot of maintenance it the weapon, then I'd say a piston operated system is in order. (Use it, throw it in the truck, forget it until the next time you need it).
Just about every make of the AR platform makes a 'lefty' but you will need to shop around.
The marvel of the AR is that the lower is the only FFL part of the system. I would suggest you put most of your starting effort into choosing the correct lower for your needs and then go looking for the upper. THere are so many aspects to be considered when choosing a lower. Fixed stock, collapsable stock, forged, billett, standard or flaired well and the list goes on, and on and... well you get the idea.
Personally, I started with a fixed position Rock River lower and built my own custom upper. I did a lot of research, handeled a lot of uppers at shows and finally though 'why not build my own?' I invested in the fixtures and specialty tools and did it my self. Learned a lot in the process too!
Good luck with it EricnGA!