Yes, you need to test all points to get a baseline. Incoming well water, post purification, and fresh made salt water.
And breeding freshwater and building a stable reef are two entirely different things. Breeding is more difficult than "keeping" freshwater, but fish have a much wider tollerance than corals. That's what this whole forum is about, and there wouldn't be a zillion posts if it was as easy as fresh water.
I have a fresh water planted tank that basically takes care of itself. Most people have a tough time getting a stable planted tank, and I did too until I purified my water...with very little husbandry, I grow plants like crazy, even with flowers blooming above the water, but that is child's play compared to a reef.
We are all telling you this because we see some red flags in what you are doing, but I feel you have the desire to learn and continue to consider comments herein. That is why we are still around. Even just the humidity issue is very serious with a reef. Hot lights = lots of evaporation = salt air everywhere...in an open style arrangement like yours, this must be considered very carefully especially with that kind of water volume.
Keep asking questions and you'll get there. I spent forever just reading threads about system design, equipment, and various issues before I dared post or start a design. We want you to be successful, so please don't take the few negative comments as anything other than a little prod to do more research. Many who have commented have been doing reefs of various sizes for many years. The cummulative experience is stupendous.
For me, you are welcome to contact me any time, but ultimately you will have to take a bunch of opinions and experience, cull it down, and develop your own plan (of course). I would suggest one giant cube, or one long "touch tank" style system like you see in public aquariums.