It is just pure water. It should be fine for using in your loop as coolant.
It is better than using tap water for sure, something I highly recommend you don't use. It is also better than using deionised water because it will have less particulates. I haven't used the Mayhems pure water but I have bought a large amount of pure water from another place and used that myself.
Yes you could use it and add biocide and inhibitor once you get them. The amount you will need to use will depend on how much fluid is in your loop. It will tell you on the bottles how much to add, usually just a little but the instructions will make it clear.
