I have built a couple of LH ARs including a 300blk pistol and have a little info for you. There are two types of lefty AR setups that I'm aware of.
Firstly, Stag has a completely mirrored BCG and bolt that rotates into the lugs clockwise. It is best to have an ambi barrel extension, although not entirely necessary. Keystone also manufactures an upper set up like this that uses Stag BCGs. The Stag BCG is made up of entirely LH parts.
Secondly there is a DPMS set up that uses a LH bolt that works with a standard BCG and rotates into the lugs like a regular AR, it has mirrored only the bolt face with extractor and ejector switched and simply ejects out the left side of the weapon. This is the setup that RedX builds too I believe.
These two systems are not interchangeable with the other.
I prefer the second set up myself. It uses more standard parts that will be easier to fix and replace than the Stag set up. It was also going to be a real pain in the ass to have a stag barrel extension fitted to barrel. At the time Stag didn't sell pistol barrels to the public. I ignored the ambi extension and ran 5.56 through a standard extension on a DD barrel and it worked fine. They say it will wear out the bolt more quickly but I haven't seen it.
The only drawback that I have found with the DPMS type bolt married to a standard BCG is that the firing pin retaining pin is open to the elements, Under heavy use it has backed out until it contacted the upper causing the gun to jam. Not a big deal at all and was easily remedied. In a righty gun the pin is on the opposite or inside of the BCG than the ejection port so it can't walk out.
My current lefty-fav is a slick sided 300blk pistol with the DPMS set up. She's a real sweet shooter. You'll have fun either way. Life's too short for lefties to run righty guns.