First, where are you getting your discus from? Are they wilds or are the locally bred or are they imported from Asia? Do you plan on purchasing a pair that has already formed or do you plan on purchasing 5-6 individuals and letting them pair by themselves? Knowing these things will help determine the water parameters that they need, not only to live, but to reproduce and the tanks you will need.
It is true that if the water contains too much calcium and magnesium that your fertility rates will drop off as will the viability of the egg as a result of osmosis becoming more difficult and the ability to exchange gases across the membrane becomes much more difficult as well. However, we are talking very high amounts of calcium and magnesium and you most likely won't have that in your tap water. I have bred discus in a pH of 8.0-8.2 and a GH of well over 300ppm and have had 95%+ fertility rates.
Wilds require water that very closely approximates the environment from which they came: soft and acidic. Domestically bred discus will thrive in a pH range of 5.5-8.5 and they would prefer at least some hardness to the water. Anywhere from 50ppm(considered soft) to 300+ppm (often referred to as liquid rock) of calcium and magnesium will work for them and they will be capable of breeding in this as well.
The fry also need hardness to the water and many discus breeders keep it around 100-140ppm to promote healthy bone structures. The most important thing however is clean water and frequent feedings. If you aren't prepared to feed at least 5-6 times per day then you shouldn't even consider trying to grow out discus fry. Hatcheries in Asia change 90-95% of the water 2+ times per day while others implement a continuous water change system.
Also, there really isn't such thing as a "small discus breeding project". You should have at a bare minimum a 29g tank to permanently house the pair (with no other inhabitants), a 20g QT tank, 3 29g growout tanks, 2 55g growout tanks, and a final tank or two about 75-125g in size.