Hi Matt - I'm new to the forum, so I'll just share my personal experience with you. I keep 15, 4" Discus in my 125g and they all fit nicely in my tank. If your budget allows, I'd suggest 10-12 Discus in your 120g. If you get your Discus smaller than 4", I wouldn't suggest just dropping them into the 120g tank - they may get swallowed up and feel unsafe. I've always heard that they should be kept in groups of 6 (at a minimum), and the ideal number would be 10-12 or more (depending on tank size). I'm not 100% sure if I'm going to be adding tank mates aside from corydora's. If I didn't need some bottom feeders to clean up after the big guys, I don't think I would have even been adding them at all. I prefer the look of a Discus only tank =)
As far as plants, (please note that this is my personal opinion), I've had bad experiences in keeping my water immaculate while having a planted tank - I do not plan to plant this tank, aside from maybe a couple of potted plants (after my QT period is over).
82F is on the low end of the temp range for Discus - I'd aim for 84-86F. The pH sounds great but remember that a stable pH is most important- a fluctuating pH can harm the fish (using pH up/down to hit an exact number)....
Hope I helped!
Amanda