Hi, what sized tank?
They are not that hard to keep when they are mature - for someone starting off its better to pay more and get decent sized discus, a minimum of 8cm which should cost in the vicinity of $80 each, rather than get smaller ones which need daily large water changes and are more prone to disease and sickness (well thats my opinion anyway). It also depends on whether its a bare bottom tank or a planted tank. You really need a minimum of 50 litres water per mature discus and at least 4 discus or a breeding pair.
Discus are peaceful fish and hang in the mid - upper levels of the tank so as long as you don't put them in with fish that zip around creating havoc you should be okay. Best tank mates imo are cardinals, harlequins, neons (that sort of fish). I also have bn, sterbai corys and rams in with mine.
The other thing to consider with tank mates are the tank temperatures which should be around 28-30 degrees for discus as some fish can't handle this temperature. I find with the neons and harlequins that their life span is somewhat reduced due to the higher temperature which speeds up their metabolism.