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You might add some better test kits to your wish list. Salifert is a good one, but I did a cost/performance analysis between Salifert and the new Red Sea here:
http://www.njreefers.org/showthread.php?75413-Salifert-VS-Red-Sea-Reef-Foundation-Pro-Test-Kit
But, as you mentioned you should get a...
One meter two probes:
http://www.bulkreefsupply.com/store/products/monitors-controllers/hm-dgital-tds-meters/dual-inline-tds-meter-dm-1-hm-digital.html
Yes, you should know what the readings are before and after the DI unit. Otherwise, you won't know the condition of your membrane. A dual in-line TDS meter is the easiest, with the probes before and after the DI unit.
There are many ways to configure a sump and/or refugium, and just as many opinions on which one works the best. I think you're at a stage where a little reading/study would be in order, and then you can decide which way you'd like to go. Here are a few articles that will help...
Why should you use RO/DI water? by Marc Levenson
When getting into the hobby of marine tanks, the basic consideration is water. Without it, nothing in your tank would live for long. Compared to freshwater tanks, marine tanks need excellent water quality to...