Discus are so rewarding... so expensive... and so demanding. I don't want to discourage you, but I want to be realistic.
You must buy full size adults. You will not be able to raise juvies in a tank with other species or that is planted. Juvies are TONS of work, and in practical terms require a bare bottom tank with daily water changes and feedings 3-4 times a day.
My second concern is how many angels are we talking about? a 55 isn't that big, I am not sure you have room for discus, or that they will be able to sucessfully compete for food with the angels. If your angels pair up it will be even worse- then your discus are in big trouble and will be tourtured incessantly by the pair in a tank that small. I tried keeping the two species together in a 125 and it was impossible. The discus ended up getting their own 90 within a week of bring them home and the difference was night and day- Discus that had been shy and had muted colors, never seemed to have full bellies became bright eyed, showed amazing colors and would eat out of my hand. I really don't think you can do both in a tank less than 1/2 the size of the one I tried.
What's your tap water like? If its not really soft, can you afford a RO system that will produce enough water to do the necessary water changes and have someplace to store that water?
Finally, plants are a great idea for a tank with ADULT discus, but you mentioned you wanted to do plants becasue discus need soft water, and I just want to explore that comment a little. Plants will not soften water. Okay, that's not exactly tru. They will take up some Mg and Ca (which is part of the general hardness measurement) but not in any real quantities. If you have high light Co2 enrighed high tech planted- then it will be measureable, but likely still insufficent. They will have no effect on Kh.
They are messy eaters, so if you do end up getting rid of the angels and purchasing full size adult discus, a sand or other very fine substrate (like ecco complete for example) is helpful since uneated food will sit on top. Otherwise half of what you feed them will end up down in the gravel, only making it necessary to gravel vac it out on a daily basis.
They are the most spectaular fish in the world when done right, but there is nothing more sorry looking than a stunted unhappy discus- to say nothing of the time and money wasted. And that is what you will end up with if you don't give them 100%. There can be no cutting corners. I hope you do get them someday when you are ready to give it your all. Maybe that day is now, but think about it long and hard. You'll see threads here about a lot of people who tried and failed, even the ones who thought they could do it- I saw one just the other day titled something like discus waterchange burn out.... after 6 months of having to do it EVERY DAY who could blame them?