Only have a moment so I'll pick ONE item and explain.
RO/DI is Reverse Osmosis / Deionized water. It's HOW the water is filtered and the ONLY way to be sure your reef water is PURE to begin with. It's more of an investment and the
ROI on it is noticed in 3-6 months depending on how MUCH water you use. If you don't do RO/DI now you WILL later and odds are you MAY have water quality issues until you do. It's much easier (and cheaper) to go ahead and get it from the beginning and eliminate one source of likely problems from your reef tank.
Good luck and TAKE A BREAK from reading
It's about ENJOYMENT!!