I think you'll be fine with regard to temperature, but if you are getting juveniles, please read up on discus. They require frequent feedings and water changes to avoid stunting. Try here:
http://www.simplydiscus.com
Keeping discus in a 40g can be done, I just wouldn't recommend it. It is preferable to keep discus in schools, which is difficult in small tanks.
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Eheim Pimp #254, Eheim Wolverine #1
55 Gallon Work in progress
10 Gallon Shrimp Tank
10 Gallon Planted QT
20 Gallon Shrimp Tank (Work in progress)