To touch on a couple of your questions...
1. As KOTD mentioned, with discus that size, you may want to do at least 2 water changes per week, imo.
2. I also agree bare bottom would be ideal in that situation, and sushiray's suggestion, imo, is a great solution as a compromise, while you are growing them out.
3. Unless you are planning on getting wild discus... which I would not recommend for a beginner anyway... you do not have to worry about your pH. Just make sure it is stable. Tank bred discus are not sensitive in the same way wild caughts are, regarding pH. Your bigger concern here is to not have it fluctuating, and I think that is best served by leaving it at what it is, and buying quality fish from a quality source. I happen to know people breeding discus in hard water and a pH of 7.8.
4. Yes... if you can afford the adults, those are easier for beginners, because you can put them in a planted, and you can do less frequent water changes. adults are a bit more forgiving of mistakes than juvies are. so if your budget will allow it, then you want to go with adults.
If you are interested, send me pm, and I can give you some other info as well, that may help you.
-Rich