I agree with pink and lupin as far as stocking goes.
I will add go with the largest one you can afford this being your first attempt at keeping discus. This will make it easier for you, not easy just not as difficult.
Juvenal discus need PERFECT water ie daily PWC of 50% or more and feeding 5 x a day hence the wc's. It is very easy to stunt Juvenal discus.
where as larger discus will not need this amount of care.
82 is the lowest I would go I keep mine at 84
Last thing keeping discus is all about the water more than almost any other fish the better the water the better the fish.it's not just a saying its fact