i would recommend a school of no less than 5... or perhaps 7. I tend to go with odd numbers for schooling fish since i started, now i can't recall if there was a reason for it or if its just my preference. Discus tend to have aggression towards each other as one of the members here stated, there's always going to be a runt of the pack that doesn't grow quite as well. from what i've read, the bigger the school the less the discus will bully each other. they're really quite amazing.
bare bottom tank is better since its easier to clean, discus seem to be sensitive to subtle changes in the water chemistry so you need to be on top of water changes as stated by other members here. you'd probably also want to invest on a water chemistry kit. i have one by api, it was about $30 bucks.
if cost is your concern, you'd better start saving, depending on the breed, a 4 inch discus will run you about 40-50$ starting. it's pretty big money up front but once you get the hang of caring for discus it's really rewarding.