MD's water is better than mine. Still he has problems with BGA on his discus tank. This is a sign of issues with his water. It only requires slight issues to cause these problems. The reason why I recommend RO/DI is so you can have soft water and control what goes into it. You start with a clean slate.
I have one of those white 105 litre water containers that I occasionally top up every night. Overnight I can make about 50 - 70 litres. I decant the water from this from time to time for top ups in my tanks. If I'm doing a water change I'll fill it full for use later on.
My house is naturally warm, so even though fresh RO water is cold, over the course of a few days it warms up to the surrounding temperature. In my setup it's about a 3 - 4 degrees colder than the main tank temperature (I'm running 25 celsius for my planted tank), the fish don't really mind, it only pulls the temperature down about 1.5 degrees considering the percentage of water I'm changing.
Since you're only doing 40 or 15 litres a time I'd just boil some RO water in a kettle and use it to raise the temperature in the mixing container. If you need your storage container to be warmer consider cladding it in insulation or something. Again this is a question of quantity, so just use some common sense.
If you really want to raise temperature on a water storage tank then I would look into getting baulkhead fittings so I can plumb a pump line to the container (my water storage container comes with one as standard that I don't use), you can then run a circulation pump and buy one of those inline heaters, so you won't need a glass heater in the storage tank touching the plastic with the possibility of melting it. It's for this reason I don't recommend thermofilters like the Oase since inline heaters are available - why waste filter media space? Personally I don't like heating large water storage containers because it can cost a lot in electricity.
Water change on my 360 litre planted tank consists of draining the 105 litres out of it and then pumping the 105 litres from the water container via a eheim 1048 pump. 30 minutes tops. I then add the water hardeners, GH and KH. That's it.
RO units you can get anywhere, you can even ask your local aquatic shop. I use RO Man or Osmotics. Units are less than £100 if you want a non under sink model. I use RO water for drinking and cooking purposes too.
