Hi Raul, with my limited experience keeping Discus:
Q1 API pH test (or just get the Freshwater master) for TDS HM Digital TDS-4 or EZ
Q2 You need a tub for aging and heating water
Q6 most people that had success recommend 50% daily
Q7 yes you need to cover your tank, Discus can jump out if they get spooked and sometimes they do it for no reason
But Raul, if it is your first time keeping Discus, for the love of humanity don't start with juveniles. You can mess them up pretty bad if you don't know what you are doing. You should start with adult fish, I know that it costs alot more, but that is the best thing you can do if you are new to Discus.
I hope other members can answer your other questions