I go by 1 adult discus per 50 litres. Its better to get adult discus if you are new to keeping them as they are easier to look after, particularly if you have a planted tank. The smaller the discus the more often the water changes and the cleaner the tank needs to be kept. My tank is 450 litres and I have 6 discus plus other smaller fish in it. I do water changes once every 2 -5 days depending on the tank readings but try to do 2 a week of about 50%.
There are top quality discus up her in auckland at good prices. Read up on them, its a matter of personal choice which type you get. Some will remain the colour they are when you purchase them, some types like pigeon discus will pepper(get black specs over them) if you have a dark background to your tank or lots of dark areas)>