water ager works, so does letting the water sit for a while with strong aeration . personal preference i guess?
as far as getting the water to the same temp as the tank goes, i personally just use cold water from the tap through a water filter, but refill over about half an hour so theres no sudden temperature drop, only gradual. i do roughly 40% at a time and get 3-4 degree drop in temp at the most.
alot of people do however get the water to the same or similar temp, allows you to do larger waterchange, and faster.
you have probably realised by now that there is a heap of contradicting advice when it comes to discus, best to read/listen to everyones point of view then do what works for you
definately not extremely hard to keep at all, and you are in the right city to get nice sized fish at a fair price