aw shush postman i'm not all that much of an expert, you're making me blush.
rainwater has virtually NOTHING in it (P and N in the parts per BILLION range from acid rain) and if you go that route you have to add calmag plus to it.
I use city tap water and don't treat it with anything, it's fine. But yeah the flouride is not the best...
Um what else... I've never grown big bud and my recommendations for easy but solid strains for a nOOb are nothern Lights, pure Afghani, and skunk #1.

The NL and Skunk have a rep for being solid genetics AND taking lots of abuse.
And the Afghnais that I have dealt with are as a group hardy, sturdy plants with gorgeous broad leaves and a very sweet perfume/spice aroma. I was using Afghani Dream in my breeding when I was doing it, and the offspring have their pics up all over this forum; there's one in foxysox's log, a whole bunch in shovelhandles af x BOG log, and another in my old 'miss stinky's first grow log'... I have no fucking clue what page it's on, but the Yeag posted it. I actually talked to him on the phone last night (well like most nights lol) and he was saying that it was the best weed that he or his friends had found in his city up to that date; better than anything that was kicking around previously. He grew his out organically and also released the genetics to a larger grower, yay for spreading LOVE. Anyway yeah. But do NOT get the afghani x moroccan that Nirvana is selling; it's WAY too sativa and IMHO that defeats the whole purpose of growing Afghanis! dUUUrrr!