Yeah, all of my hydro water is RO/DI water. 0ppm at the beginning of mixing in nutes. Hard water isn't a bad thing, but it does make achieving your target ppm a bit more "complicated". Not sure what resources you have available or RO/DI equipment. Just to let you know, I built mine piece by piece, from Bulk Reef Supply a very long time ago. Now, their prices for kits are much more market competitive and ready to go out of the box. Shop around and you may find something that meets your growing needs at a reasonable price.
The CalMag should give you an upper hand on the blossom rot. Also, don't forget about Epsom salt, simple and effective. Think about that after you run out of CalMag. No need to have them both at this point.
If you haven't thought about it, start a glog about your progress. I think other hydro growers (like myself) would definitely take interest.
I have looked up a few systems. But I feel like im out deep where im not sure what to go after. If you have some systems you could recommend that ships to europe I would like to hear them .
Im definitely planning on starting a glog, I just need to get past the last exams, I have quite the albums the share already
Im afraid you might be right, im going to see if things change, and if not, RO seems to be the way to go!