Discus do not do well by themselves. If you get just one and put it in a planted tank, it will either hide all the time or die from stress-they are a schooling fish. That being said, discus do no harm to plants, mine spawn on them. It is commonly said that it is best to start with 5-6 smaller discus in a bare minimum of a 55g.
A good resource for considering discus in a planted tank is Barron's Discus Fish book.
If you have any other questions, there are a few dedicated discus keepers here, we can offer advice or direct you to sources.
Kevin