Replacement parts for the EHEIM classic series are available online, but I agree some of the newer versions are easier to clean. They come at higher price.
You should never leave lights on more than 12 hours a day. Unless you want an algae boom. Most planted aquarium owners do not leave their lights on for more than 8~10 hours a day. Once algae cover up the plants, the plants will die because they can't receive the lights. The tank water might also turn green. Even fish will go crazy if there is daylight all the time. Try to use an automatic timer to turn on/off your lights at the same time every day.
If you are really going for a planted aquarium, I would suggest you to set your 40 gallon up, and plant the plants before start doing a fishless cycle. Plants do need a head start, in case some fish might like to unroot them.
