never heard of eheim but i know that fluval filters are the way to go!!!
and some live rock? i hope you get at least 60 pounds of it.
you need tons of hiding places for fish to make them feel safe and not be so stressed.
and remember--cycle the tank for about a month in order to get everything ready--i.e. no adding fish..add live rock and sand and let that settle with the water while cyceling.
question 1.
you really dont need to do water changes. the water evaporates so you want to ADD fresh water to top off the tank. the salinity will end up equaling out in the end because once it evaporates the salinity rises so by adding the water it will bring it back down to normal. so really if you want you can do it after about 5 months of having it set up but by all the fish and a live rock you will have they will do the work for you.
question 2.
do you really want a "finding nemo" tank. ok
clown fish are damsels and you dont really want to mix multiple fish of the same species. they will become territorial and agressive if you do end up getting multiples of the same fish. because you have 135 gallon you may be able to get 2 clown fish--as long as theres hiding places for them.
-if you want a blue hippo tang (please dont call it a dory, go by what the real names are because it gets annoying) but anyways. blue hippo tangs are good. just know if you are going to want to get another tang--say a yellow tang or a powder blue or brown tang---you will want to make sure that they have different shape bodies...(and that combination works-you can combine a blue hippo and a powder brown)
-with tangs like i said, i am going to emphasize that make sure that you find tangs with different bodies.. if they look alike that is bad news. we made the mistake of combining a powder brown tang and an atlantic blue tang together and about 6 days lader the atlantic tang died because the powder brown picked on it so much. so just be warned about that.
-i dont remember all of the fish in "finding nemo" but i am going to mention other fish.
-getting a goby or blenny would be great! my boyfriend has a mandarin-kind of shy but a great fish! a gold face sleeper goby are fun and entertaining!! they filter out the sand and its really cool to watch!!
-also go for something different, you can try a lion fish. i have one and my boyfriend has one. you just have to be careful and NOT touch it, it is deadly so if you arent worried about it then you can try that out.
-fire fish goby. they are really cool!!!
question 3:
-you can only get corals if you have the proper lighting. you will need actinic/medal halide lighting...its to copy the sunlight in nature. so if you get the correct and propper lighting then you can get corals/anemones. but just know you also need to look for fish that REEF SAFE!!! there are fish that will eat corals, for example angel fish. they will tear them up!
**EDIT:**
You will want two powerheads because one will flow the water in that part of the tank but not all threw the tank. Put one on each side of the tank in order to get really good air flow!
hope this helps and ill add more if i think of anything or if you have more questions!