10g per adult discus is the general rule, but once your used to keeping the tank running ok you can go slightly over this. If this is your first time keeping Discus I would say start with a group of 6 and see how things go.
I would not buy baby discus unless you are prepared to provide them with a bare bottom tank, and really in that size of tank you'd need to buy a very large group and you wouldn't be able to keep them in there for all their lives.
You'll find it easier if you buy fish at around 3-4" if you don't mind getting bigger then bigger the better I think, I find the larger ones settle in better and it's very very easy to stunt a baby discus. If your buying larger discus then a bare bottom tank will still provide the easiest method of keeping the water really clean. Some people don't like them though, you can keep lots of bogwood and plants on bogwood or in containers.
As for water conditions I think it's very important to mix the parameters of your Discus supplier. If you find a good discus breeder/supplier then will use suitable water conditions and by using the same you are avoiding stress as they are moved. As long as the water is on the soft side with a ph at or just below neutral it should be fine. Look into the use of RO water to soften your water if required.
It is most important to provide constant water parameters with Discus, they don't like change at all!
