Avoid any LFS discus unless you know exactly where they get them, I don't care how healthy they look. In the end there is a very high chance you will have problems with them. With Discus you pay for what you get, and you need to look long and hard into the reputation of the places your getting them from and get good solid advice from people that keep the fish. If you really are serious about getting into discus visit
http://www.discusasahobby.com and start asking questions. There are many breeders and importers that are constantly on the board to help. Aquaria central is not the place to be getting good info about discus. There are very few of us that know what there is to know about discus, and many people that think they know what they are talking about when they don't.
Keeping discus is not hard, but you need to have the ambition and desire to give them the things they do need.
Dave, your right Discus are not hard to breed. However what you failed to mention is that raising quality fish from egg to sellable fish is not an easy task. It takes a lot of time, effort, and extremely good knowledge of the fish, as well as the ability to get input from others who know the fish even better than you. Discus are a totally different animal when it comes to raising fry than 99% of the other cichlids out there.
As to the tank raised tank mates suggestion, you will not encounter anymore problems with wild caught cardinals or cories for that matter if you follow the correct Quarantine proceedures, which you should be doing no matter the source of the fish.