RDWC system advics

I'm going to take your questions in reverse order here: #5 - don't buy it. As other have said, you'll not have a good time of it. I would rather see you purchase something like the bucket system I use where each plant has it's own reservoir - since different plants react differently to nutes and may need different things at different times, this is the best system for a non-commercial grower. They have a 1-, 4- and 8-bucket system. https://smile.amazon.com/Hydrofarm-Culture-Bucket-System-Gallon/dp/B01326JYTG/ - and it's much cheaper than the one you're considering. The only thing you're going to have to do is to drill a small hole in the top so you can withdraw water to test the EC and pH or to add water/nutes. Cover the hole with something like duct tape so the bucket remains completely dark. #4 - If you've never grown DWC before, go easy - it's quite different. #3 - Check your EC and your pH once a day at the same time of the day... these can fluctuate during the day so making sure you're checking at the same time of day is important #2 - You'll have to get a good sense of what each of their nutes does to both pH and EC... what you add, in your hypothetical situation, will depend on what stage the plant is in... #1 - I usually do a complete change when the plant goes from veg to flower and then about 2 weeks before I anticipate harvest... as long as the pH remains stable and the EC is where you want it to be, there's no reason to do a complete change although SOME growers make that complete change once a week when in flower. Good luck...