Help- RO/DI unit

I am in the process of setting up a 90 gallon reef tank. I have everything I want to start with, except a good RO/DI filter. I have never had one before and can't say I understand what I want, but have found a website that is highly regarded (so it seems), less expensive, and allows me to design it my way. I plan to have softies and LPS coral, as well as fish. I am looking to get off to the right start with the right RO/DI unit. Here is the website, it just asks you to select 5 parameters and check a couple options. What should I get? I am also on a budget, but realize the water is probably the most important component in this aquarium. The website is : Build Your Own RO System - English

Thank you for your help to keep these fish and corals healthy and happy.
 
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