Welcome to Canreef! (Or welcome back to canreef?)
If you use an auto top off and look into a herbie style return the sump should be very quiet in terms of noise. The loudest thing in my sump is my return pump's fan on the back, and that is very quiet. I'm the same way with finishing the stand, don't see the need to cover it, it works well as a stand, making it look pretty won't help me :P.
I noticed that if i leave my RODI off for a day or 2 the lower water level in the sump causes some water noise with the water comming over the baffles, but if i leave the RODI on the water level stays up with little noise.
Obviosuly the draw back to all in tank systems is the clutter that builds up, and limited options for expanding. I've been using biopellets on my new tank and i will probably never NOT use them again, i love those thigns so far. There's some nice hang on back refugiums around, but I don't see a huge benefit from a small one hanging from the tank, i like the bigger one in the sump.
I'm not sure about the Tunze skimmer, the 9020's rated for 500+ gallons, so should be able to handle it. I've been using the COMLINE calcium reactor and been very happy with it. It's half the size of most and i'm barley stressing the thing, It should have no problem as my corals grow.
Here's a nice page for skimmer comparison's if you havn't seen it yet:
http://aquariumcontrollers.com/files/skimmers.html